A dramatic proposal made two weeks ago to the nation's governors by Japanese-American businessman Kay Sugahara to help shore up the U. S. economy with $10 billion in low-interest Japanese loans has never been taken seriously by the Japanese government or Japanese businesses that supposedly were backing it, according to knowledgeable Japanese officials and executives.

They say, in effect, that there is no plan or fund--simply an idea being promoted by Sugahara. He had claimed that the aid proposal, offered in part to compensate for the toll taken here by Japanese imports, had the backing of the Japanese "power structure."

In an interview here yesterday, Ahido Amaya, a special adviser to the Japanese Ministry of International Trade and Industry, said that the Japanese government "has never been informed" of Sugahara's project and that he does not believe anyone in the Japanese business community "has given it any serious consideration."

"It's just a private idea," Amaya said. "And it's not only very ambiguous, but very large. Given that combination, it sounds very unrealistic to me. Maybe he mentioned it to someone in an informal talk who might have said, 'That's a good idea.' But we don't think there was a serious discussion."

Other well-informed Japanese label the idea of offering concessional aid to the United States "an arrogant one." Added one official who was here last week to discuss trade matters: "In the U.S.-Japanese relationship, Japan is the disciple and the U.S. the mentor. So this whole thing is unacceptable." Said another: "Our problem is that Sugahara has a Japanese face, but he's an American."

Within a day after Sugahara outlined his proposal Feb. 22 to a meeting of the National Governors' Association here, he began to amend the description of it as a reverse "Marshall Plan," saying instead that the $10 billion would be not just loans but a combination of loans at low interest and investment capital, possibly in conjunction with American investments.

Sugahara, who had suggested that an organization he heads here be a kind of clearinghouse for projects undertaken through the fund, said that after further talks with potential contributors, he had agreed that in some situations it would be better to think in terms of investments than loans.

But a Reagan Cabinet officer said: "I'm afraid there's nothing there. I'd like to see the first $1 billion, let alone $10 billion." He suspects that the Japanese government is furious because the Sugahara proposal has provided a convenient measure--the $10 billion--by which Japan might try to reduce its politically embarrassing trade surplus with the United States.

Florida Gov. Bob Graham, chairman of the governors' association's international trade committee, had met in advance with Sugahara. Graham had then endorsed the proposal in a Feb. 5 letter to fellow governors, which said that the "concept" had been reviewed by administration officials who saw it as "a significant opportunity."

Over the past two weeks Graham has refused to return phone calls from this newspaper, which ran the first story on the Sugahara plan. A press secretary read a prepared statement by Graham that said the Sugahara plan, although at a very preliminary stage, "is a serious offer which deserves to be explored further."

In a lengthy telephone interview last week from a vacation retreat in Jamaica, Sugahara was asked to respond to the many disavowals and denials coming out of Japan. He brushed them all aside, saying repeatedly: "You have to understand the Japanese mind."

He claimed again that he has all the financial backing in Japan that he needs, but refused to identify individuals or companies, except to say that he has solid commitments from 25 influential Japanese exporters "who have a stake" in avoiding a major reduction in their sales to the United States. "If it the plan goes good, they will all surface," Sugahara assured.

"The reason that they Japanese critics don't know about it is they're not at a high enough level," Sugahara said. " My Japanese colleagues don't want any publicity until the governors come up with their list of projects. They tell me, 'Kay, find the formula and find the volume of financing it will take to change the climate.' "