Assistant U.S. Secretary of Commerce Frank A. Weil complained yesterday that the prices of American goods in Japanese' markets are not declining as fast as could be expected in light of the rapid appreciation of the Japanese yen.

"There is some evidence that Japanese importers are not passing through to the Japanese consumers the full effect of the appreciation of the yen," Weil said.

As an example, he cited American-made automobiles whose prices he said have declined here between five and 10 percent in a period when the yen has appreciated by more than 30 percent.

Weil began a series of talks with Japan's trade experts in a continuing U.S. effort to boost the sales of American-made products in Japan in hopes of reducing the U.S. trade deficit with this country, which stood at a record $6.6 billion at the end of June. The deficit was twice as high, in dollar terms, as it was at the same point last year.

Weil also said he and many observers feel it is unlikely that Japan will attain a 7 percent growth rate this year, a target which was set - and is still maintained - by Prime Minister Takeo Fukuda.

In a news conference and luncheon speech, the assistant secretary laid heaviest stress on the failure of the yen's rapid appreciation in the past 19 months to substantially lower prices of U.S. goods here.

American expectations have been that the yen's rise and the dollar's decline would result in U.S. goods becoming cheaper in Japan and that the volume of their sales would markedly increase.

The actual results, however, have been uneven and generally disappointing to U.S. officials, although many, like Weil, have acknowledged prices would fall only in the long run as a delayed reaction to the yen's rise.

According to figures from Japan's economic planning agency, retail prices of U.S. autos fell about 10 percent on the average in Japan between December 1976, and April 1978, a period in which the yen appreciated by nearly 30 percent. Refrigerator prices declined by about 19 percent.

American businessmen often complain that it is Japan's rigid and costly distribution system, supported by decisions made in government agencies, which prevents price cuts from being passed through to Japanese consumers.

Gentler in his criticism than are many businessmen trying to penetrate this market, Weil said the reasons for the failure of prices to drop must be studied closely in coming months. One reason, he said, is simply that, "businessmen hate to lower their prices." A lack of competitiveness between importers is another, he suggested.

On the other side of the coin, he said, the prices of Japanese products sold in America have not been raised to levels equal to the yen's appreciation. If the changes on both sides do not begin to approximate that appreciation, he warned, "we will then be faced with a continuation of these imbalances which will produce consequences that nobody is terribly happy about."

Weil also warned that "there is again a rising sentiment about Japan-U.S. trade balances" in Congress which could lead to a new round of protectionism. He referred to a proposal by Rep. Charles Vanik (D-Ohio) and other congressmen to impose a 15 per cent surcharge on Japanese goods. There is no "serious intention that I am aware of" for the executive branch to go along with that proposal, he said.

"But there is a growing sense . . . that some protectionist measures may at some point be required," he added. "The executive branch wants to resist those protectionist notions because we know that it could trigger a whole whirlpool of problems that might take us back to the problems of the "thirties that served nodoby's purpose at all."

Weil's assertion that Japan's goal of 7 per cent economic growth rate this year is probably "unattainable" came as a surprise to some, because the Fukuda government has steadfastly maintained it will keep that promise he made at the recent Bonn summit meeting.

The current forecasts by private research firms here place the growth rate at a lower figure, some as low as five per cent, based on the economy's record so far.

However, Fukuda has promised a new round of economic stimulation measures to be decided upon in early September to give a new boost big enough to reach 7 per cent.

Weil, in his speech to foreign correspondents, said: "For this year, for Japan, I think a realistic assessment of all observers is that it is so late in the year that it's very unlikely that that particular number can be achieved.

But he added, the United States is encouraged that Japan is willing to try. "Having that (7 percent) as a goal and taking steps to achieve it is what is important," he said.