Japan is looking back over its shoulder, and what it sees is an embryonic economic giant nipping at its heels from across the Korea Strait.

The Japanese worry about the prodigious economic leaps made over the past 25 years by its former colony of South Korea, the pace-setter of the "four tigers" of Asia whose export-led high-powered growth until last year's slump has been the envy of the rest of the developing world and many industrial nations.

"The Koreans work so hard" is a common refrain from Japanese officials, diplomats and industrialists who appear from this side of the Pacific to be no sluggards themselves.

Korea is the most likely of those Asian tigers to become the next Japan, a prospect that sends shivers that spread from Tokyo and Washington to centers of American high technology and the headquarters of traditional manufacturing industries such as autos, steel and textiles.

Koreans bristle at being tabbed the next Japan. Culturally, they don't like being compared with their former conquerors; practically, they think it will hurt their economic drive, which is based on selling more and better products overseas.

Korea still remains far from being an economic superpower like Japan; its per-capita income is just $2,000, one-fifth of Japan's.

"It is my view that most of the [U.S.-Korean] trade frictions stem from a misconception regarding Korea's economic policies and its role in the world economy," said Kim Kihwan, secretary general of the Korean government's International Economic Policy Council in a speech last year to the U.S.-Korea Society in New York.

"In some quarters," Kim continued, "Korea is regarded as a 'second Japan,' a country bent on flooding the world markets with its exports while jealously protecting its own domestic markets. This view is, of course, not only inaccurate but unfair."

Korea has grown into an economic force through hard work and the successful application of a game plan that started in the early 1960s to emphasize exports. Since then, Korea's overseas sales have leaped from $52 million in 1962 to about $30 billion in 1985, with 90 percent of them consisting of manufactured goods.

Like most developing countries, Korea first concentrated on the simplest industries -- textiles and clothing -- which rely on a plentiful supply of cheap labor and whose machinery or capital investment needs are light. While these industries still form the backbone of Korean exports, the country has moved far beyond its simple industrial roots.

Its construction and engineering teams moved into the Middle East more than a decade ago, winning contracts with bids far lower than those of established European and American giants.

With its Pohang steel complex, Korea is considered among the most efficient steel producers in the world. Its electronic firms have captured from Japanese manufacturers the low end of the American market for television sets and video cassette recorders, first as nonbranded items but now proudly bearing the names Samsung and Goldstar.

These companies are moving into inexpensive clones of International Business Machines Corp.'s personal computers and, by the end of the month, Korea will unveil its first car in the United States market, a $4,495 bottom-of-the-line Excel made by Hyundai Motor Co. that was a runaway success in Canada last year.

The coming of a Korean car attracted the attention of Americans, whose view of Korea until now largely had been formed by M*A*S*H, the hit movie and TV series. Politicians, labor leaders and industrialists in the United States are beginning to realize that they face an import invasion from a second Asian nation that could rival Japan.

The word has gotten around Capitol Hill: It may be too late to stop Japan, but the United States can't afford to let Korea get that kind of a toehold here.

This view disturbs the Koreans, who see themselves as the innocent victims of a U.S. backlash against Japan that could cost them a large chunk of the American market -- responsible for buying more than one-third of their exports -- before they can gain a firm hold on it.

Moreover, any closing of American markets threatens Korea's economic growth, which slipped last year as a result of a fall-off in the rate of increase of its sales in the United States. Korean economists figure the country needs at least a 6 percent yearly growth rate to keep pace with new entries in the labor force.

"While Korea's economy has grown at a phenomenal rate, the country is also in a particularly vulnerable position with regard to protectionism. Any U.S. initiative to close its market to Korean goods could be disastrous to my country," said Hee Jin Kim of the Korean Traders Association in a speech here last year.

Faced with the greatest challenge to its game plan for speedy economic growth, Korea has become one of the shrillest voices in attacking what it sees as the new protectionist trade policies of the United States. These include Reagan administration efforts to put more teeth in laws against unfair trade practices while attempting to get the Korean government to open its markets to American products.

But Korea's views of protectionism go beyond the international norms. Koreans, for instance, list as protectionism the penalty duties imposed as a result of their unfair trade practices, such as illegal government subsidies or dumping goods in the U.S. market at less than their cost of manufacture.

Although its industries have gained greatly from President Reagan's veto of a textile quota bill and his refusal to follow an International Trade Commission recommendation to limit shoe imports, Korea complained about a dumping case involving 8,000 workers making photo-album covers.

While it agreed under heavy U.S. pressure to limit its steel sales, Korea is free to sell its cars and electronic goods -- which have the heaviest value-added component -- in American markets.

At the same time, the Seoul government of President Chun Doo Hwan sees nothing wrong with its own strongly protectionist policies, which include arresting Koreans caught smoking foreign cigarettes in public.

Korea also blocks sales of foreign-made small computers, chocolates, beef and life insurance, among other products where American manufacturers and service industries feel they have a competitive edge, and it insists it needs more time to pass laws that will preserve foreign books, chemical formulas and other intellectual property from being pirated. Students became so concerned that their supply of cheap, pirated textbooks would end that they rushed to bookstores this fall to stock up.

Radical students took over an American Chamber of Commerce office in Seoul last year to protest American trade policies while President Chun called efforts to enforce limits on imports and efforts to open Korea's markets "dual shock waves" to the country's economic growth.

Most Koreans think America is picking on their country, and they assert what they consider the "blood ties" from the Korea war as proof of a special relation that should allow extra privileges.

Lee Ki-hong, publisher of Korea Business World, expressed this view during a press conference by Sen. John C. Danforth (R-Mo.) in Seoul last month when he asked "why Americans are singling out Korea" for trade actions.

Then Lee answered his own question: "America has been penalizing Korea for its successful economic growth."

"Korea was opening its market for [the] mutual benefit of our two countries," Lee continued. "This is not the case with some of our major trading partners such as Japan. We have the same problems [with Japan's closed markets] and have been suffering from a chronic trade deficit even with the United States. Only recently have we had a very modest trade surplus" that could be written off if interest on Korea's loans were included.

"Regardless of what you have said, Koreans still feel that they are being picked on," Lee concluded.

" . . . If anybody is picking on anybody, it is not us picking on you," replied Danforth, chairman of the Senate Finance Committee's trade panel. "In 1984 . . . Korean exports to the United States increased 29 percent. At the same time, our exports to Korea increased only 2 percent . . . "

Danforth also carried a strong message to the Koreans. If Korea doesn't want to be considered a second Japan, it had better stop following the Japanese model of economic development that includes "keeping out all of the products it can keep out other than what it absolutely needs" and "targeting industries, using licensing measures . . . to keep out the products of other countries," he said.

"Is it going to follow that model?" Danforth asked. "Or, instead, is Korea going to be devoted to the concepts of free trade and fair trade?"

The warning was simple: Congress won't stand still if Korea continues on the protectionist path. "We, as American politicians, are going to be standing up for the prosperity, the hopes, the dreams of our own people," Danforth said.

Oddly enough for a country that covers its trade relations with America in almost limitless detail, the press coverage of Danforth's speech was "scattered, very light and generally disappointing," according to a report by the American Embassy in Seoul.

This was due to "a conscious nationalistic desire by all Korean media to play down presentations of direct American criticism of the country's trade practices or defenses of American policy positions."

"Getting a message to Korean publics [sic] they don't want to hear is no easy task," concluded the embassy report.

Which means that some of the claws of the Korean export tiger may be trimmed this year by a U.S. Congress more concerned over job losses among constituents than the economic growth of even as close an ally as Korea.