IT ISN'T OFTEN that the layer of courtesy which the Japanese usually show to Americans is pulled back to reveal the real feelings, the scheming and the flashes of arrogance, that sometimes lurk beneath.

That is why a document now making the rounds of U.S. government offices and some corporate suites is considered a "hot" item -- revealing enough, in fact, for a major Japanese company to have helped prevent Outlook from reprinting most of it today, shortly before Japan's new prime minister arrives for a visit in the United States.

The document, an interview with the chairman of Japan's leading computer manufacturer that was printed last year in a respected Tokyo magazine, sheds valuable light on Japan's tactics in its fierce struggle with a U.S. corporate giant -- IBM -- for world technological supremacy.

It tells why and how powerful Japanese computer companies scratched to get their hands on secret information about IBM computer designs. "We had to know exactly what IBM had up its sleeve" so Japan could produce computers that could use the IBM software programs dominating world markets, said 70-year-old Taiyu Kobayashi, chairman of the giant Fujitsu, in the interview. Hence, he said, reliable information about IBM designs were "funneled" to Fujitsu from a U.S. company founded by a former IBM computer designer.

It tells how the Japanese government moved to protect the Japanese market for Japanese-made computers. "Before we even knew whether they'd run or not, MITI (Japan's Ministry for International Trade and Industry) was there helping us out by getting after the auto makers and steel companies to start using the domestic makes" instead of IBM computers, Kobayashi remarked.

It also tells about the peculiarly Japanese view of the ethics surrounding some business practices, including the ones upset last year when Hitachi and Mitsubishi Electric Co. were charged with conspiring to steal IBM secrets outright.

While Kobayashi said the criminal charges were "really terrible," his primary concerns apparently were not about questions of honorable actions or criminality under U.S. laws. Rather, he worried that the incident damaged "our image," strengthing the view that "Japanese enterprises are crafty and play dirty . . ."

Indeed, throughout the document Kobayashi and his interviewer actually see IBM as the villain, seem surprised by American officials' fury over Japanese protectionism, and are puzzled by American worries about advanced U.S. technology ending up -- via Japan -- in Soviet hands.

The Kobayashi interview -- conducted by free-lance journalist Soichiro Tahara and originally published last September in the Tokyo monthly Bungei Shunju -- clearly does not square with the polite and seemingly accommodating stance of Japanese Prime Minister Yasuhiro Nakasone. Nakasone has gone out of his way to assure the Reagan administration that Japan really wants to be more cooperative about the undervalued yen that makes American goods more expensive in Japan, about restricted Japanese markets for U.S. exports and about Tokyo's lagging defense buildup.

It is perhaps not surprising, then, that Fujitsu would not wish the interview to appear verbatim in The Washington Post before Nakasone's scheduled visit with President Reagan on Jan. 18. Less than 24 hours after Bungei Shunju gave The Post tentative reprint permission, the magazine changed its mind. That was after Fujitsu, which learned of The Post plan, had voiced its objections to the Tokyo magazine.

"Fujitsu suggested that the complete version not be printed in The Washington Post," confirmed a spokesman for the New York public relations firm of Ruder & Finn, which represents Fujitsu in this country. The Ruder & Finn spokesperson said that Kobayashi does not question the accuracy of his quotes in the article.

There is also little doubt about why the Japanese found it imperative to get their hands on IBM designs as swiftly as possible.

When Japan began building a computer industry from scratch in the early 1970s, it faced enormous obstacles. To close the technological gap, three computer groups were established, and the Japanese government took steps to protect the fledgling industry.

However, it soon became obvious that building computers was only one problem. Even if Japan could make models as good as or better than IBM's, IBM's domination of the market had made its machines -- and the programs written for them -- the international standard. Japanese customers -- the intended base for the new industry -- had already invested millions of yen in the IBM software.

Japan's response -- the same as that adopted by a number of U.S. competitors of IBM -- was to build computers that could use the programs written for IBM models. By selling these machines less expensively than IBM, Japanese corporate strategists believed that they could wrest some of the market away from what Kubayashi calls the U.S. computer "Gulliver."

In fact, the strategy enabled Fujitsu to displace IBM as the leading seller of computers in Japan by 1979. Two other companies, Hitachi and Mitsubishi, also adopted the "IBM compatible" strategy.

The problem was that to compete successfully, the Japanese required early, detailed knowledge of the internal design, or "architecture," of each new IBM model. Unless the "IBM compatible" companies could bring their machines to market reasonably quickly after each new IBM model was introduced, customers would simply continue to purchase from IBM.

Hitachi relied for some of this information on a U.S. consulting firm, Palyn Associates, whose president was a former IBM employe. In September 1981, according to affidavits on file in federal court in San Francisco, Palyn president Maxwell Paley tipped off IBM that he believed Hitachi might have in its possession confidential IBM documents.

It was Paley's tip-off -- characterized by Kobayashi in the interview as "traitorous" -- that led to an FBI undercover operation and charges that Hitachi, Mitsubishi and 18 of their employes conspired to steal IBM secrets, such as the designs of IBM's newest and most powerful computer, the 3081K. Those involved have denied the charges.

In the Kobayashi interview, the Fujitsu chairman suggested that Fujitsu had been more clever than other Japanese companies and had avoided such risks.

"It is not a good idea to obtain secret information from IBM directly from IBM," he said. "You are just asking to be done in. But in the United States, individuals have the freedom to move about wherever they wish. If a person moves to another company, the secret information that he carries in his head goes with him, and there is nothing anyone can do about it."

In the early 1970s, Fujitsu began providing financial backing to Gene Amdahl, a brilliant computer designer who had played a leading role in designing IBM computers from 1952 to 1970 and who just resigned from IBM. Amdahl, an ambitious entrepreneur, set up his own company to produce powerful machines compatible with IBM software programs.

In 1974, Fujitsu introduced its M series of machines, intended to rival IBM's 370 model. In 1975, according to a spokesperson for Amdahl Co., in Sunnyvale, Calif., Fujitsu also acquired 32 percent of the U.S. company's stock. Fujitsu now has a technology-sharing arrangement with Amdahl and supplies semiconductors and computer subassemblies to Amdahl in the United States.

To Fujitsu, the alliance was natural, Kobayashi explained, because "Fujitsu was groping around, desperately trying to figure out how to put its hands on, or how to buy, IBM's architectural concepts." The time was also ripe for the alliance, in Kobayashi's view, because IBM was bogged down in a major U.S. antitrust suit "and was hamstrung. It was in no position to retaliate."

Gene Amdahl himself left the Amdahl Co. in 1980 to start another computer firm.

In December 1981, three months after IBM announced its 3081 K, Amdahl Co. announced its own 580 Model, to be compatible with the software programs written for the new IBM computer. Then, in June 1982, Fujitsu announced that it was ready with its own version of Amdahl Co.'s 580, the Fujitsu 380.

A company spokesperson said last week that Amdahl "does not copy IBM machines. Our architecture is different from theirs, and so is the way we process instructions."

Compatibility with IBM, she said, is possible because IBM follows certain principles of operation. "There are only so many paths that you can follow in developing these technologies. You can pretty much guess what the new architecture is going to be. Once we have knowledge of the IBM software interface, we make ours compatible."

Despite all the Japanese efforts to get hold of IBM information, a recurring theme in the Kobayashi interview is that IBM and the U.S. government are the villains of the piece, determined to "discipline Japan."

In Kobayashi's view, IBM moved against Hitachi and Mitsubishi when IBM was freed by the Reagan administration from the shackles of its antitrust case.

As for the U.S. government, he professes to be surprised and pained at criticism leveled against Japan during last July's financial summit meeting. "It would not have seemed strange to even have bombs tossed at us, that was how threatening the atmosphere was."

What moral did Japanese freelancer Soichiro Tahara draw from Kobayashi's revelations about the FBI's "sting" operation against Japanese computer companies and the tactics used by Fujitsu? Again, probably not the same lessons as Americans would draw.

Summing up the interview, Tahara states:

"Japanese feel that when they are hitting someone they are in a fight, and when they are shaking hands they are friends. But Westerners use footwork in which they shake hands with one hand and hit with the other. Hitachi's losing out to Maxwell Paley, a consultant on contract, is a clear example of the drawbacks of Japanese-type one-handed boxing . . ."

The Japanese actually are hoping that they won't have to do the same kind of fighting with IBM in the future. In the view of Kobayashi and his colleagues, a new era of smaller computers that don't rely as heavily on IBM programs may provide an important opening for Japanese ingenuity. Japan is single-mindedly pursuing small computer development and is pouring money as well into research on futuristic new computers capable of thinking like humans.

But as long as the two countries are locked in a coldblooded struggle for dominance in the next generation of technology, it seems doubtful that America and its Japanese allies are likely to reach any meeting of the minds soon on the rights and wrongs of business ethics.