Japan is groping for a way around a pension crisis that is expected to hit with full force in the 1980s and 1990s when an unprecendent number of older workers will be leaving their offices and factories for retirement.

It is made up of the same parts as the cloud that hangs over the American Social Security system: Too many retirees and not enough workers to support them without paying exorbitant taxes.

In terms of hard numbers, it is no more severe than the American problem, but it has come on the Japanese with a bewildering suddeness, the product of a swiftly declining death rate that has given the Japanese man the world's longest life span.

In just a few years, in fact, Japan has gone from boasting of its lifetime employment guarantee to a rapidly spreading concern that many elderly will be going jobless.

"It is the speed with which this is happening in Japan that is the issue," observes Haruo Shimada, an economist and consultant to the government's economic planning agency.

"It has been building up for many years in the United States and Great Britain, but in Japan, it has come to a point all of a student. The year 1985 will be the beginning of a real crisis here."

Japan's lifetime employment concept, in which companies guarantee their workers jobs barring extraordinary circumstances, was easily fulfilled for a long time. It was fixed in the early years of this century when companies began enforcing the retirement of all those aged 55 - at a time when the life span for the average man was anly about 45 years.

But that has changed. The Japanese man's lifespan is now the world's longest, about 73 years, and Japanese women live an average of 78 years, longer than in any other country except Sweden. Much of that lengthening span occurred in a 15-year period, 1960 and 1975, when Japan's death rate fell by 36 per cent.

The socially unsettling side of that coin is an aging work force and millions of people living more and more years past the age of 55, which is still the mandatory retirement age is roughly half of Japan's companies, particularly the large ones.

There will be, then, more and more people living on benefits from the two national pension systems that encompass about 90 per cent of Japanese workers. The benefits are liberal by Western standards. According to one calculation, the average monthly pension is equal to about 40 per cent of the worker's salary before retiring.

There will be fewer and fewer workers to support them. One study concludes that at the present time there are nine working-age persons for every one over 65. That ratio will drop to 7 to 1 in 1985 and 4 to 1 in the year 2010.

The burden can become enormous on the pension funds. Japan's health and welfare ministry estimates payments from welfare pension, one of the two major plans, will triple by 1983, an unusually rapid acceleration. Shimada cites estimates that by the pension systems will be on the receiving end, being paid benefits. It is only about 10 per cent now. That is not extraordinary by U.S. standards, but the speed with which it has overtaken Japan has shocked many.

A number of advisory committees, including one attached to Prime Minister Takeo Dukuda's office, has been tackling the issues for the past few years and many tentative reports are circulating through the bureaucracy. None has yet to attract much of a consensus, according to government officials, and it is not yet certain when a government bill will be submitted to the parliament.

The reform plans generally assume some kind of new tax will be necessary. One proposal is for a new social security tax to be levied on top of employe contributions. Another calls for a value-added tax on manufactured products. Both would run into heavy opposition from consumers and employes.