The Japanese government has apologized to the United States and is sweetening its regrets with what could be billions of dollars. But it doesn't want word of its expensive gesture to get out.

In an extraordinary act of governmental contrition, Tokyo has secretly promised the Pentagon it will contribute to a joint U.S.-Japanese research project that will improve antisubmarine warfare capabilities. The secret agreement was meant to assuage American wrath over the illegal sale by Toshiba Machine Co. of high-tech propeller-milling machinery to the Soviets.

The 1984 sale, which infuriated Congress and the Pentagon, allowed the Soviets to produce submarines that run too silently for existing antisub devices to pick up. It could cost as much as $30 billion to overcome the advantage the Soviets gained by this $17 million deal, and the Japanese government has tried to appease the Pentagon by agreeing to put up some of the research and development money.

According to a secret Pentagon memorandum we've obtained, the joint antisub project is being kept under wraps because U.S. officials are afraid that Japan will renege on the agreement if it is made public. The Japanese government is particularly concerned that no connection be made between the pledge of antisub funding and the Toshiba sale that made the funding necessary.

When word of the Toshiba sale leaked out earlier this year, Congress reacted with outrage.

The Japanese shrewdly sought to counter the hostility by approaching the greatest source of it: the Pentagon. In what the secret memo described as "extensive confidential negotiations," the Japanese agreed to take punitive action against Toshiba and C. Itoh, the trading firm that handled the Soviet deal.

But there was also part of what the memo called a "satisfactory settlement" that was not made public, and which, in fact, was never intended to be. The third paragraph of the secret memo reads:

"In a separate action that should not be linked in any way to the Toshiba case, MOFA (Ministry of Foreign Affairs) and JDA (Japanese Defense Agency) have agreed to create a combined U.S.-Japan ASW (antisubmarine warfare) research project to which Japan is expected to contribute substantial amounts as well as technical expertise. The goal of this project is to respond to the challenge posed by increasingly silent Soviet submarines."

Despite this clear evidence of a connection between the Toshiba sale and the new joint research project, the Japanese didn't want their largesse made known, even though it might have helped soften Capitol Hill anger.

In a revealing passage that shows the Pentagon budgetary mind at work, the memo notes that recovering monetary damages from Toshiba would be a lengthy and uncertain process, "and any funds recovered would return to the general fund rather than to DoD."

So instead, the Pentagon negotiators opted for a direct monetary apology from the Japanese to the Defense Department: the joint antisub research project.