Mike Mansfield has the best of both worlds. He is officially U.S. ambassador to Japan. But the Japanese see him as their ambassador to the United States, and both sides seem to like his way of representing them.

"He is trusted by the Japanese," said Shizuo Saito, Japan's former ambassador to the United Nations. "The Japanese people think he is influential, both with the president and with Congress. And so we think he is reliable."

In the current Washington interregnum, the former Democratic Senate majority leader from Montana seems to inspire similar approval. He is a voice in Asia that the Carter administration has listened to. And there are hints from Ronald Reagan's foreign policy advisers that he may be asked to stay a while longer in Tokyo.

In the past, Mansfield has said he will serve only one term as ambassador, ready to leave when Carter's first four years expired. Not now. Asked in an interview this week if he would stay if Reagan wants him, Mansfield replied with uncharacteristic coyness, "I'd give it serious consideration." But the big Montana smile shows he would be delighted to remain.

Since he came to Tokyo in June 1977, Mansfield's tenure in Tokyo has been an almost unblemished love affair. From the Japanese point of view, he is a heavy-hitter -- a man with influence back in Washington, sent to a country that often feels ignored by its principal ally. For the Japanese, he has been more a mediator than a diplomat. He has occasionally chided them gently on such matters as trade and defense, telling them what they ought to do for their own good, but his great value to Japan has been in straightening out Washington's errant ways.

Last December, for example, former secretary of state Cyrus Vance was quoted in Paris as calling Japan insensitive to the hostage issue in Iran because the Japanese allgedly circumvented trade sanctions and bought high-priced Iranian oil. Mansfield investigated and found that the Japanese government was not undercutting the sanctions and had already stopped the private oil purchases.

Did he protest? "Very vigorously, I sure as hell did," Mansfield said this week. "Because they were wrong."

There have been other instances in which Mansfield's way clashed with Washington's. A persistent view in the White House and Pentagon has been that Japan will not do anything right unless its knuckles are rapped. Mansfield's touch is softer.

"I think that if you want Japan to do what we want them to do, we should let them do it in their own way and not threaten or demand or pressure," he said.

He thinks Japan has, in fact, supported U.S. positions more firmly than European allies in the cases of sanctions against both Iran and the Soviet Union.

"Of all our friends, they have been the most supportive. They've also paid the biggest price. For example, when they refused to pay that $2.50 extra for [Iranian] oil last March, two days later it [the export of Iranian oil] was cut off. It's still cut off. That's 13 percent of their imports."

Mansfield favors a steady increase in Japanese defense spending and says so publicly, but he also feels that this still largely pacifist country cannot be pushed too hard.

Others in Washington think Japan should be goaded to increase its defenses dramatically and quickly. One day Manfield found a hawkish Republican Senate staff member on his turf in Tokyo pushing the lhard line. A phone call from Mansfield got him sent home.

A more telling example is the recurring dispute between Mansfield and Secretary of Defense Harold Brown. Mansfield has disapproved of American attempts to fix exact percentage targets for the Japanese military to meet.That was clearly the Carter administration's policy until Brown showed up in Tokyo with a list of percentages Japan should reach by 1984.

It irritated Mansfield. Asked this week if that sort of breach had been systematic, he said, "No, in the beginning it was. But it's wrong to place emphasis on percentages. It's the substance that counts." He paused, then added, "The Pentagon doesn't do that anymore."

The Japanese consider Mansfield, 77, a genro -- a wise elder who acts from many years of experience. Many of Japan's political leaders are in their sixties and seventies and feel comfortable with a man of his age. Government leaders appreciate mostly his connections back in Washington.

"They say that it is very important for an ambassador to have a direct channel with top officials in the [American] government, and from that point of view, I think Mansfield is number one," said Masahisa Naito, an official of the Ministry of International Trade and Industry.

Mansfield deals with the Japanese as he did with his old colleagues in the Senate, slowly and patiently. His enjoyment of the job is total. He is at his desk by 7:30 most mornings, before most of the embassy people have arrived, and he is erect and alert, although signs of fatigue are likely to show by late afternoon. His speech is blunt and direct, and he rarely speaks off the record with reporters.

Picked as ambassador after a brief retirement in Florida in 1977, the former Senate majority leader says he has never regretted leaving Washington. fHis greatest joy, he says, is "just getting away from the manifold pressures in Washington." In Tokyo, the problems come one at a time for Mansfield, and there is more opportunity for slow consideration of each.

"The Senate was getting to be a terrible headache," he said. "I could not even begin to keep up with it. Since I've gone, it's gotten much worse." nHe cites the growth of single-interest groups and lobbyists as the main problem of managing Congress.

"Even Sam Rayburn and Lyndon Johnson could not have handled the House or Senate the way it's developed in the last 10 years."

One of his regrets is the too-successful movement he helped lead to weaken the presidency and strengthen Congress after the war in Vietnam.

"I think we went too far," he says now. "It ought to move somewhat toward the middle with the president having more power than Congress but not too much."

The ambassador looks back on his country critically. Americans have grown complacent, he believes, and the lack of civilty bothers him a great deal. He puts it all into a few succinct phrases:

"Too many fat people. Too much crime. Too little security. Not enough pride or courtesy or politeness. Too many people for themselves. Too willing to 'let George do it,' but they don't want to be personally affected." w

Japanese, on the other hand, hang together as a people. "Deep down, they know how vulnerable their country is strategically, economically, and when the going gets tough . . . they get in and produce more -- because they have to. They have pride in what they do. It pays off for their country. I think we ought to have pride in what we do."