Japan's trade barriers, which once made this country a citadel of protectionism, have fallen dramatically in the past decade to the extent that some experts now consider Japan a substantially open market for the world's goods.

Many quotas have been abandoned and tariffs are falling. Government monopolies are being forced to let foreigners compete in markets once rigidly protected, and many of the less visible trade obstructions are being liberalized.

The effect of a decade of trade liberalization has been a marked increase in the sale of foreign manufactured goods here. But American businessman are finding it tough to compete in the opening market. Their share of the trade in many manufactured goods declined through most of the 1970s and only recently have they begun holding their own against other foreign sellers.

Recognition of these falling barriers came recently from an unexpected source. A task force of the U.S. House of Representatives trade subcommittee, which in the past has been critical of Japan, concluded that although some protectionist attitudes remain, "Japan today is generally an open trading nation."

"It has become increasingly clear to us, and to many businessmen dealing with Japan, that our trade problems result less and less from Japanese import barriers, and more and more from domestic American structural problems of competitiveness and quality," the task force, headed by Rep. James R. Jones (D-Okla.), reported.

American businessmen trading here cite many lingering obstacles. But a bilateral trade group that includes many U.S. importers in Japan concluded that a long-range turn-around in the unfavorable U.S. trade balance with Japan depends more on better American industrial performance than on eliminating the remaining barriers on this end.

Japan was the world's most protectionist industrial state during the 1950s and 1960s, as its government rigidly obstructed imports in order to nurture domestic companies in the eriod of high economic growth. It erected a powerful array of quotas, tariffs and government monopolies, along with an elaborate maze of bureaucratic techniques that kept out foreign goods.

But many of these obstacles fell one by one in the 1970s, under pressures exerted by irate trading partners who saw their own industries being overrun by Japanese cars, television sets, textiles and other products.

By the end of the 1970s, Japanese agriculture was the only major sector still protected by quotas, and tariffs had been reduced substantially. The Tokyo Round of multilateral trade negotiations calls for further tariff reductions -- so much so that after its eight-stage agreement is fulfilled in 1987 Japan's average tariffs will be lower than those in the United States and Europe.

The U.S. congressional report noted that problems still exist for agricultural imports and for several Japanese government monopolies that pursue a rigorously "buy-Japan" policy.

However, the resistance of government monopolies currently is falling, too. By the end of this year, trade negotiators believe, the major monopoly offender, Nippon Telephone and Telegraph, will open a large part of its annual $3.2 billion in procurement to foreign bidders.

The NTT case is regarded as especially significant because of the large sums of money paid for communications equipment, a field in which American suppliers are considered extremely competitive.

NTT, a government-controlled corporation, had strenuously resisted permitting any foreign bidding. The resistance crumbled this year, however, when Congress passed legislation preventing Japanese bids on U.S. government procurement unless the NTT case was settled by Dec. 31.

Japan's government tobacco monopoly also is expected to open this country's doors to U.S. and other foreign tobacco suppliers by the end of this year. That virtually would eliminate barriers erected years ago by government corporations that had been particularly visible violators of international trading customs.

Despite the changes, American companies have found it hard to compete with those of other countries for the Japanese markets. In a number of fields, European and third-world competitors moved in with more competitively priced products.

A study by an American firm, Arthur D. Little Inc., found that between 1968 and 1975 U.S. imports to Japan "grew increasingly noncompetitive with imports from other countries," although that trend began to turn around in the late 1970s. For example, the U.S. share of Japan's electrical machinery imports declined from 72 per cent in 1968 to 54 per cent in 1978, and there were similar lapses in fields such as transportation equipment and precision machinery.

Two authories on Japan trade, James D. Abegglen and Thomas M. Hout of the Boston consultin group, noted the declining American competitiveness in an article two years ago and attributed it to a failure of U.S. business to take advantage of the openings.

"Too few U.S. companies are prepared to pay the price of entry, in large part because too many U.S. companies maintain an out-of-date view of Japan," they wrote.

Some American businessmen here blame their problems on a variety of import obstacles loosely classified as "nontariff barriers." These range from erratic customs valuations to special taxes to import classification and certification systems.

One expert who represents several American exporters here cites as a major barrier the low-level administrators who think they are supposed to interpret regulations in a restrictive manner, regardless of what the regulations seem to say.

"The customs man is supposed to interpret the law, but will actually do what he thinks his superor wants, not what the law says," the trade consultant said recently. "This is most true of lower level bureaucrats. They choose the most conservative standards they can apply. The easiest way is the conservative way."

For nearly three years, a group of private and official Japanese and American experts have met regularly to review allegations of unfair treatment against American goods.

Initially, the group, called the Trade Facilitation Committee, was almost flooded with 90 separate complaints from American businessmen claiming discrimination. After a preliminary review, however, all but 21 of them were discarded as unwarranted and officials say most of them merely reflected misunderstandings. Six of the 21 have been resolved through negotiations.

Some U.S. businessmen still regard these barriers as irritants, while acknowledging that they are not a major factor in reversing the overall trade deficit with Japan, which will reach about $10 billion this year.