Ambassador Mike Mansfield said yesterday he believes Japan has made a genuine tournabout in its foreign trade policy and is committed to a plan that will encourage imports and reduce its trade surpluses.

Mansfield, the former Senate majority leader, also said a recent visit in Washington convinced him that the rise of protectionism in the United States has been "stilled," largely because of the Carter administration's new policy of trimming foreign steel imports.

In a news conference at is residence her, Mansfield was optimistic about the course of trade relations between Japan and the United States. The Japanese have made "sacrifices" to satisfy demands that they open up their markets and their change in policy is not just a matter of "cosmetics," he said.

The burden now, he suggested, is on American businessmen to try seriously to sell in the Japanese market on a long-term basis and not just attemp to move in quickly to make a "fast buck."

There has been a renewal of septicism here about Japan's real role since the agreement which was signed in January cooled off a tense confrontation with the United States.

A new round of private economic forecasts has predicted that Japan will not reach a 7 percent growth rate, as projected in the agreement. Early reports on trade balances also cast doubts on Japan's ability to reduce its surplus by very much this year.

Mansfield said it is too early to tell whether Japan will hit those targets, but he added with considerable emphasis:

"We have got to give them a chance and I think we have got to recognize that if we downgrade everything they are endeavoring to do, if we become cynical and pessimistic, then we are certainly not helping them to achieve their goals."

He said he thinks the Japanese "have had to make some real sacrifices. I think they are determined to do what they can, but this is a two-way street. Our people will have to do what they can."

Asked what "sacrifices" the Japanese had made in the agreement, he singled out the promise to import more beef and agriculture commodities, principally citrus fruit. He acknowledged that those increases were "small in numbers," but said their real significance was in demonstrating the government's willingness to make them in the face of fierce hostility from domestic farm lobbies.

Another sacrifice, he said, was Japan's pledge to work for tariff reductions and other changes in the multilateral trade negotiations in Geneva.

Mansfield's view of declining protectionists forces in the U.S. Congress has been widely discussed here and has come as a distinct relief to the Japanese, who had feared they might be shut out of a large number of U.S. markets.

In the ambassador's opinion, the center of protectionism was a large powerful caucus of senators and representatives from steel manufacturing states who wanted tough restrictions on Japanese exports. They represented the chief power behind protectionist legislation designed to raise tariffs or impose tough quotas on many items in addition to steel.

The Carter administration's new trigger-price system of curbing Japanese steel has considerably lessened the force of those pressures, Mansfield said yesterday.

Mansfield also said he expects to see "concrete business results" from a highly publicized mission of Japanese businessmen who left Tokyo yesterday for a tour of American cities. The mission's main goal is to convince American businessmen that Japan can become a major market for them and the Japanese government hopes it will help remove the country's stigma as one which keeps out imports by devious means.

U.S. officials have tried to convince that mission it should place specific orders for goods while in the United States. So far, the mission's leaders have said the trip is primarily an exploratory one designed to explain Japan's import policies and to suggest which goods might be sold here in the future.