TOKYO, DEC. 20 -- Responding to U.S. demands for a bigger contribution to the cost of stationing American troops here, the Japanese government today announced that it would gradually increase its share to about 50 percent from the current level of 40 percent.

The decision is aimed at satisfying U.S. complaints that Japan should do more to defray the expense of keeping the 50,000 troops on Japanese soil. Congress has become adamant that allied countries, especially rich ones like Japan, should provide more military "burden-sharing" at a time when the United States is struggling to pay its bills.

Chief Cabinet Secretary Misoji Sakamoto stated that Japan had "voluntarily" decided to increase its contribution, because Tokyo's mutual security treaty with Washington "continues to be a solid foundation of Japan-U.S. relations."

{In Washington, a State Department official said, "We welcome Japan's decision. Japan already provides the most generous support of all the nations where our troops are stationed."}

While Tokyo is pledging to spend more for the cost of U.S. bases, it is sharply slowing the growth rate of overall military spending. The cabinet today approved a five-year, $171 billion defense plan that envisions annual budget increases of about 3 percent, considerably below the 5.4 percent increases of the previous five-year plan.

That move is likely to be cited by Japan's critics as evidence that Tokyo still is not shouldering a big enough defense burden, especially because Japanese military outlays will fall back below 1 percent of gross national product -- a level it exceeded beginning in 1987. Following World War II, Japan agreed with the United States to hold its military spending below 1 percent, but was pressed by the Reagan administration to breach that ceiling as a way of sharing more of the defense burden.

That increase was regarded as a potentially significant expansion of Tokyo's military role and, while it muted criticisms by U.S. conservatives, it raised concerns among Japan's neighbors. The decision to allow military outlays to slip back below the ceiling could evoke fresh accusations that Japan is shirking its defense responsibilities while enjoying the fruits of its economic miracle.

But Japanese officials said the slowdown in military spending is only natural, given the thawing of the Cold War. "The Soviet Union was a potential threat for Japan in the past, but we now believe that that potential threat . . . has disappeared," Sakamoto said at a news conference.

The increase in the amount Japan plans to pay for the U.S. troop presence will be phased in over five years, and will require an outlay of $600 million in 1995 above the previously planned level, a Foreign Ministry official told reporters. Japan's contribution currently stands at about $3 billion a year.

The Japanese pledge goes most of the way toward meeting the U.S. request for Japan to pay the "yen-based" costs of the troops' presence -- that is, almost everything but the troops' wages.

The increase will involve expenses for electricity, gas, water, sewerage, heating fuel and wages paid to Japanese personnel working for U.S. forces. Garbage collection and telephone expenses, however, will not be covered by Tokyo.