A leader of the ruling Japanese Liberal Democratic Party yesterday suggested creation of "an economic alliance between the United States and Japan" through which American companies would be permitted to borrow money in Japan at rates "substantially lower than in the United States."

In a speech to the Japan Society in New York, Zentaro Kosaka, former foreign minister, and presently chairman of the LDP's foreign affairs research council, offered this proposal as a practical way of defusing current trade tensions between the two countries. He said it would help the United States to "revitalize" industry, which he identified as the key American economic problem.

Kosaka said, in response to questions, that the money would be advanced by "private organizations, commercially based," rather than by the Japanese government. The government's position, he said, is that such an idea is interesting and that he had been commissioned to make the suggestion and to assess the response to it in this country.

In urging restoration of harmony between the two countries, he implied that Japan would for the first time approve large-scale American borrowing within Japanese financial markets at low interest rates, if the idea could be made politically acceptable to the Reagan administration.

It was put forward, in a frank way, as preferable to liberalizing Japanese restrictions on agricultural imports, which he said would cause political problems for the LDP "and aggravate public sentiment toward America," or to major boosts in Japan's defense expenditures.

Kosaka, a member of an influential Japanese family, is an elder statesman widely respected in government and business circles. He is still active in the Japanese parliament, having been a member continuously since 1946.

Kosaka said that under the plan he was suggesting, no interest rate subsidy would be involved, because the borrowing would be done "at market rates in Japan." Currently, these rates run about 10 percentage points below American rates. He ducked questions on whether direct investment by American companies in Japan, now legally permitted, would also be welcomed.

The trial balloon Kosaka floated as a way of providing low interest rates would entail "flotation of bonds in Japan by some American enterprises" to provide an incentive for productivity.

"I am aware that there must be certain practical limits to the extent such operation is feasible," the Japanese official said. "But I am offering this suggestion for your attention as an example of various cooperative opportunities existing in our economic alliance." He refused to mention any dollar figure.

The Kosaka plan was clearly a follow-up to one advanced about a month ago by Kay Sugahara, an American businessman of Japanese parentage who talked of collecting $10 billion from the Japanese private sector for low-interest loans to finance projects backed by the 50 state governors. But there was an effort to divorce from the Kosaka version any suggestion of a "reverse Marshall Plan." This was considered offensive in official circles here and in Tokyo.

With Sugahara in the New York audience, Kosaka referred to the businessman's plan in an approving way yesterday, saying its "purpose may not have been fully appreciated."

Separately, it was learned that Foreign Minister Yoshio Sakurauchi had had a guarded conversation with U.S. Trade Representative William Brock on the subject of low-interest loans on his visit to Washington two weeks ago.