AT A TIME when the U.S. auto industry is getting further mired in depression, the question of booming foreign car sales here -- especially Japanese cars which now dominate sales -- has touched off a new debate in Washington and in Tokyo.

One out of every six new cars sold in the United States is made in Japan, a fabulous growth from 1973 when the Japanese were selling one in every 16 in the American market.

There's no mystery about it: The Japanese and other small-car exporters have offered American buyers the fuel efficiency they have been looking for (as well as more advanced design in many cases) in an era when Detroit had refused to face the energy facts of life.

American companies are in the midst of crash programs to reorient their lines to smaller cars, but for the moment they can't supply the demand. In some U.S. metropolitan areas, customers must wait in line for six months or more to buy popular Honda Civics or Volkswagen Rabbit diesels. There is one alternative that would reduce pressure to restrict Japanese imports -- a decision by the two big Japanese producers, Toyota and Nissan (which makes Datsuns), to follow Honda's lead and build assembly plants in this country. Together Toyota and Datsun sold one million cars here in 1979.

BUT THE LARGE and conservative Japanese companies, fearful of head-to-head competition with the U.S. auto industry (especially General Motors Corp.), are restricting the move. They foresee a situation two or three years down the road in which plants they might build would come on steam just as GM reaches full capcity for its own small cars.

Toyota and Nissan, anxious about the risks of international investment, would rather stay in Japan, secure in the hands of a protective government, dealing with a more compliant labor force and assured of their own quality control. Toyota, in fact, is building a huge new plant near its headquarters in Nagoya, Japan -- a capacity which is ticketed primarily for American sales.

Rep. Jim Jones (D-Okla.) thinks there's another reason for the Japanese reluctance to invest in the United States -- a fear of getting caught up in U.S. politics. So far, 38 states have bid for a Japanese auto plant, and neither Toyota nor Nissan wants to get whipsawed by competing interests.

One official with good Japanese sources says that California Gov. Jerry Brown, hearing that Nissan at one time was considering a $200 million investment in the United States, told the Nissan board: "If you don't put that plant in California, there'll be retribution -- remember, one day I'm going to be president."

Honda is more innovative, and already is thinking of boosting from 120,000 to 200,000 units the capacity of a $200 million plant in the United States.

In an attempt to find some solutions to these vexing problems, Rep. Charles A. Vanik (D-Ohio), chairman of the House Ways and Means subcommittee on trade, will start hearings on March 7 on trends in world trade in automobiles. Vanik said in an interview that he's not ready to prejudge the question of "whether legislation is needed to limit imports." But one item on his committee's agenda reads as follows; "The impact of imports on the domestic industry's employment, cash flow and ability to modernize."

There are plenty of hawks on Capital Hill ready to slap either import quotas or an import surcharge on Japanese cars. Worried by such developments, the powerful Japanese Trade MINISTRY (MITI) is sending Vice Minister Naohiro Amaya to Washington on an official visit just before the Vanik hearings begin.

Japanese officials are beginning to test the water to see what position to take. MITI officials are ready to press the Japanese companies into a "voluntary restraint agreement" (VRA) that would limit their U.S. sales to the 1979 levels, provided that American officals endorse such a VRA as a good approach.

JAPANESE COMPANIES complain bitterly that they are being asked to suffer for managerial blindness in Detroit that resulted in American investment in gas-guzzlers when the rest of the world knew that the era of cheap energy was finished.

But the Japanese companies and the government in Toyko got together years ago to shut American cars out of the Japanese market totally, and U.S. trade negotiators never have been able to pierce Japan's high tariff walls on cars.

The prospect of any U.S. limits on Japanese imports chills those officials of the Carter administration worrying about energy and inflation. "What do you gain by limiting car imports?" asks a trade official. "Since there aren't enough American fuel-efficient cars to replace the imports, all you're doing is pushing buyers into bigger cars that they don't really want."

But the complaints of auto union president Douglas Fraser on the threat to American jobs is a powerful force on the other side of the issue in an election year.

As the Japanese assert, there is as yet no coherent thread to American policy on this issue. The president's trade representative, former Florida governor Reuben Askew, is supposed to be taking the lead. But now that the big Japanese companies are resisting Askew's invitation to invest in this country, Askew and his staff cannot make up their minds whether to support "voluntary" restraints on Japanese cars.