The Japanese government is convinced that automobile exports to the United States must be restrained, by government decree if necessary. But so far the government has been unable to convince Japan's automakers of that.

For weeks, the government and industry have been locked in a kind of underground debate, bits and pieces of which surface occasionally to suggest that the consensus so highly valued in Japan has not been reached.

On one side, the government is saying that keeping industrial peace with the United States is more important than selling more automobiles and that the Japanese auto industry must restrain its exports if it does not want American protectionists to impose a quota on them.

On the other side, industry objects to the idea of government-ordered restraints and claims that, anyway, the laws of economics will reduce exports of Toyotas and Datsuns this year. Nobuji Araki, executive vice president of Toyota Motor Sales Co., Ltd., stated his company's bottom line today by commenting, "We are still a public corporation with a responsibility to our shareholders."

Japan's Ministry of International Trade and Industry has been urging the big car manufacturers for months to exercise restraint. The minister, Rokusuke Tanaka, told the Japanese parliament today that a form of "administrative guidance" might be applied to encourage that restraint a bit.

A few years ago, when the ministry was very powerful, "administrative guidance" was a threatening phrase that often brought industries quickly into line. But it does not carry that much weight any more. Araki said, during an interview, that his company would judge the American scene objectively on the facts.

"We look at the situation, not the words of one minister," he said.

For the public record, Toyota and Nissan Motors refused to talk about the question of "voluntary restraints," the phrase being used by the Reagan administration as possibly the best solution this side of quotas or quasi-quotas such as marketing agreements. They fear voluntary restraints among themselves smack of a cartel and believe they would trigger antitrust suits in the United States, possibly by their own American sales agents.

They also contend that the laws of laissez-faire economics will solve the problem anyway. Araki observed that the defuge of Japanese cars in the American market last year occurred at a time when the yen was at a low point, which made their cars cheaper.

The yen is much higher now, he said, and Toyota has had to raise prices twice this year already to take account of it. Besides, he said, the new American small cars will be coming on the market soon and will complete with the The Toyotas.

"There will be a natural decrease," Araki said. "The problem may resolve itself."

The evidence for that is not yet convincing. Japanese car exports to the United States did decline in late 1980 from levels of a year earlier. But in January they were up about 16 percent from the same month in 1980. The government contends that for the first three months of this year exports to the United States will be 2.4 percent lower than in 1980.

Until the Reagan administration decides how to handle the issue, Japan apparently will delay giving its final response. Quiet talks are under way between the Trade Ministry and the industry. If any agreement has been reached it is a well-kept secret. Tanaka, disclaiming well-publicized reports of friction, insisted today that his ministry and the auto industry are "on very good terms."

Both the industry and government have been comforted lately by signals from the White House that President Reagan will oppose legislated quotas or even a so-called "orderly marketing agreement," which is a quota of limited duration. In private comments, industry leaders have indicated they sense that the protectionist sentiment may wane in time if Reagan rejects the views of some Cabinet members who want legal limits put in place.

The problem for the government is how to arrange a system of voluntary restraints among the automakers if, in fact, the final decision at the White House is to ask for them. There have been suggestions that the solution could be something like the trigger-price system now enforced on the steel industry, prohibiting sales of Japanese steel below a fixed price.

For the Japanese government, the issue is coming to a head at the worst possible time. Prime Minister Zenko Suzuki is to visit Washington late this spring and would like to avoid a head-to-head argument over car exports.