Sen. Pete Wilson (R-Calif.) cited what he called "an old Japanese proverb" in a toast last night to Japan's trade minister, Michio Watanabe: "Two pilgrims will not succeed in climbing Mt. Fuji if one has to carry the other."

"If Minister Watanabe has trouble recognizing the proverb," Wilson said, "it is because I just made it up."

Wilson's sally drew appreciative laughter from an after-dinner audience of California exporters and trade officials from Canada, the European Community, Japan and the United States.

But it also touched a theme that ran through a two-day meeting of ministers from those four giants of world trade: the need to get Japan to buy more foreign products for both its own sake and the sake of the world trading system.

The EC's minister of external relations, Willy deClercq, said there is an "urgent" need for Japan to lower its burgeoning trade surpluses with practically every country in the world. "The lack of equilibrium causes great concern and great nervousness," deClercq said at a news conference. "The discontent with the situation in the European Community is the same as in the United States."

U.S. Trade Representative Clayton Yeutter echoed that view at a press briefing. "It is important that the Japanese government give an increased thrust to imports. Without it, it will continue to be challenged by all the governments of the world," he said.

"It behooves the Japanese to do something dramatic" to cut their burgeoning trade surpluses, Yeutter added.

That message forms the major thrust of President Reagan's trade policy with Japan in the face of a trade deficit with that country that probably was close to $50 billion in 1985. It also is the rationale behind the year-long series of bilateral talks to open Japan's markets to specific products.

The Reagan administration called the talks a success, but Yeutter said Japan still hasn't opened its markets enough. "We still have a long way to go," he said.

Prime Minister Yasuhiro Nakasone, who agreed with Reagan last January to start the market-opening talks, is promoting what is known as an "import vision" in Japan to push the purchases of foreign goods. Even though it has resulted in some major U.S. sales -- including a supercomputer and two communications satellite systems -- the effort has not been enough to make a dent in the trade deficit.

In Tokyo last week, Sen. John C. Danforth (R-Mo.) warned the Japanese that they face protectionist measures from Congress if they don't start lowering their surplus.

Yeutter, though, said the Reagan administration is not asking Japan to set a specific target for purchases from the United States.

In talks he held here with Watanabe, meanwhile, a new Japanese policy emerged that could distort trade flows. It provides low-cost loans to small and medium-sized exporters who have been hurt by a 20 percent rise in the value of the yen over the past three months. The yen's rise makes Japanese goods more expensive in foreign markets. "We told them they can't do that," said one American trade official.

He said such government loans violate international trading rules set by the 90-nation General Agreement on Tariffs and Trade unless a country has severe balance-of-payment problems. That exemption does not apply to Japan, which has the second strongest economy in the world and a massive balance-of-payments surplus.

These low-cost loans, moreover, undermine the very reason for deliberately increasing the value of the yen and lowering the value of the dollar, which is to make Japanese goods less attractive in world markets and end trade distortions caused by the currency misalignments.