This is probably the most critical year of all in our relations with Japan." That's the sober judgment of Ambassador Mike Mansfield, whose untiring efforts to achieve a sensible compromise on all outstanding U.S.-Japanese issues has made him a revered figure here.

In an interview, Mansfield said that Prime Minister Yasuhiro Nakasone's effort to open up Japanese markets even more to foreign goods and his professed willingness to share military technology with the United States "have bought some time."

But it is clear to this reporter after a week's candid conversations with officials on both sides, and with business leaders and other opinion makers as well, that the tensions have grown, not lessened.

Moreover, what are generally considered "indiscretions" by Nakasone on his recent trip to the United States (when he displayed a hawkish stance toward the Soviet Union--even to the point of offering to make the Japanese islands an "unsinkable aircraft carrier") have weakened his popular support.

As a result, factions within his Liberal Democratic Party are already gunning for him. They think Nakasone is willing to make too many concessions to the United States on trade and economic issues.

A further complication is that the Reagan administration is now pressing what amounts to an unremitting effort to get Japan this year to beef up not only the total amount of its defense expenditures, but its true military capabilities.

Washington regards the Soviet Union as the enemy. But most Japanese, though uncomfortable with needless provocations from the Russians, still prefer to regard the Soviet Union as "a difficult neighbor."

The problem is that the Japanese government is not really sure how much of an additional military budget will satisfy the Reagan administration.

There is no longer any disposition to argue Washington's point that Japan is getting a "free ride" when its budget calls for less than one percent of GNP for defense. At the first opportune moment, Nakasone intends to try to pull the Japanese Diet into a commitment that will break the one percent barrier.

But American officials have been emphasizing to their counterparts here that merely to break into the 1.5 or 2 percent zone of defense spending won't relieve Japan of the "free ride" charge. There is talk of "burden sharing" that can be met only if Japan also expands its economic assistance abroad and undertakes vaguely defined political and diplomatic responsibilities around the world.

Japanese officials told Henry Kissinger, Helmut Schmidt, Raymond Barre and other Western big shots at a private "round table" discussion here this week that Japan will be able to move only slowly into a more active military posture. These Japanese officials are a bit bewildered by what seems an expanded American demand.

On strictly economic and trade issues, Japan still must deal with the "fairness" question. Professor Ezra Vogel, whose book, "Japan as Number One," has attracted attention here, feels that the widespread belief that Japan doesn't play the trade game fairly will lead to lots of protectionist bills in Congress.

But the public perception here is that Japan is being "scapegoated" for what are essentially American competitive failures. The Japanese are astonished by complaints such as that of Rep. Sam Gibbons (D-Fla.) here last week that Japan "targets" industries for export attack--picking a product and then marshalling forces to take it over.

The Japanese public believes that American companies complaining about lack of access to Japanese markets are simply not doing a good enough job. "That is a very powerful feeling," says Vogel. "I've been doing a lot of speaking to Japanese groups, and that's very much the mood you get in the question period."

Where does the truth lie? Vogel says that the Japanese markets are not as open as some Japanese believe, "but not as closed as a lot of Americans believe." The cutting edge of the problem is that in high-tech areas where the United States has a lead, Japanese policy still is to buy as few foreign machines as possible, then to work hard to produce something as good or better.

In agricultural goods, the protectionist power of the farm lobbies remains unshaken. After 20 years of struggling, the United States has moved only from a one percent to a 1.4 percent share of the cigarette market. Quotas on beef do nothing except line producers' pockets at the expense of the Japanese consumer, who otherwise would be able to buy cheaper American or Australian meat.

One gets the feeling that Nakasone is trying harder than any recent Japanese prime minister to shake off unfair practices. He meets some resistance in the Japanese bureaucracy and in the business world. But to survive, he will have to pacify the Japanese public that fears his hawkish trend in military affairs.