For 42 of his 57 years, Yoshiro Yamada has worked for Sumitomo Heavy Industries here, and by the normal rules of Japan's liftetime employment system he has another year to go.

But Sumitomo's shipyards in this port city are in trouble and the longterm prospect for new orders is bleak. Yamada's name is one a long list of older workers who are being asked to retire.

Unlike most of his coworkers, Yamada is resisting. Four times in the past five weeks his boss has suggested that he retire voluntarily like the others. Four times Yamada has refused. "I will die if I leave the company now." He says gently, meaning that he would consider his useful life ended. "I cannot die."

Although the resistance of Yamada is uncharacteristic, his story is similar to that of thousands of Japanese workers whose companies are having hard times these days. Japan's famous lifetime employment system is still intact but it is being frayed around the edges, and the experience is a wrenching one for workers and management alike.

Lifetime employment traditionally has meant every worker is guaranteed a job until retirement age, usually in the late fifties. It has accounted for the stability of Japanese companies in the boom years and for the remarkable loyalty of their employes, secure in their belief that even in bad years the company will take care of them.

Now that confidence is waning. Companies resort to various euphemisms to veil the layoffs. In the case of Yamada and his Sumitomo coworkers, the practice is called "Kata tataki," which means a "tapping on the shoulder." It is a much-dreaded phrase that really means: "Please leave, you are no longer needed." another device is "window-siding." The unwanted office employe is placed in a remote corner of the room near a window, far removed from where important business is done. He is given little or no work to do and eventually leaves in humiliation.

Other tactics are even less indirect. Some of the resisting Sumitomo shipyard workers here are surrounded and jeered at by large groups of their fellow workers who belong to a company union that has agreed to the Sumitomo employe-reduction plan.

One such scene was tape recorded and played back to a visiting reporter recently. An employe named Morita was seated at his lunch table when, at a whistle signal, about 150 workers gathered around him and shouted in unison, over and over: "Morita -- quit the company! Morita -- quit the company."

Such incidents are rare in Japan's prosperous postwar history and were unheard of in the high-growth days of the 1960s, but the 1970s have brought change.First there was the oil-shock-induced recession that depressed many industries. Unlike their counterparts in other industrial countries, Japan's corporations tried desperately to avoid layoffs. Elaborate plans were made to "loan" employes to healthy industries, and to a remarkable degree they were successful in averting mass firings.

The economy is now rebounding and although Japan's unemployment rate of 2.06 percent is high by Japanese standards, it is extraordinarily low by the standards of some other countries.

More recently, however, something worse has happened: Structural longterm depression in some big industries, Japan's steel and ships are increasingly uncompetitive with those produced in other Asian countries. Ship orders that once went to Sumitomo now go to South Korean yards. Those industries are being reorganized in Japan, and workers are being stripped from the payrolls. The number of employes in big-firms -- those having 500 or more workers -- has declined by 2.1 percent in the past year.

The big firms have cut back on recruitment of younger workers. Women employes, who rately work past their mid-twenties anyway, are urged to leave even earlier.

The ax is failing most frequently on male workers in their mid-fifties, those just a few years short of regular retirement age. Since most Japanese workers are given regular annual salary increases regardless of merit, it is the oldest who are paid the most money, Getting rid of them saves the company more than would a reduction of the younger labor force. Between Dec. 26, 1977, and Jan. 8, 1979, more than 7,800 shipbuilding industry workers were retired early.

Increasingly the unions are protesting, particularly the Sohyo Federation of socialist-oriented unions, which are more outspoken than the docile company unions.

"Of course, the wages of older workers are very high so it seems natural for the companies to get rid of them," says Ataru Shoji, director of Sohyo's employment bureau -- "but this is bad for society as a whole. The social costs are very high. There are high unemployment insurance costs and welfare costs and finding other jobs for them is very difficult." Japan's population is aging swiftly, Shoji observes, and the problem is compounded by a growing proportion of older workers in the labor force. "So what the companies want to do contradicts the social needs of our time."

Some observers see the companies and older employes on a collislon course that may produce the country's first serious postwar labor clash. For while the companies are trying to reduce the number of older workers and lower the de facto retirement age from 57 or 58 to 55 or younger, a countermovement is gaining support in the Japanese parliament. Legislation introduced by opposition parties would prohibit dismissal of workers younger than 60.

Japanese management executives insist that what is happening to older workers in depressed industries is a fringe phenomenon that does not seriously dilute the promise of lifetime employment. It is still the custom to hire employes for life and frain them within the factories, they say. "Lifetime employment is not being basically changed," says Yoshinobu Matsuzaki, an official of the Japanese Federation of Employers.

It is considered an embarrassment in business circles for a company to be accused of presuring for early retirements, and Sumitomo Heavy Industries denies that it is doing so. It emphasizes it is offering the older workers larger-than-usual retirement bonuses and help in finding other jobs if they leave now. Its plan was accepted by the shipyard company unions and, even according to socialist union sources, only about 10 percent of those asked to leave have resisted here.

Shunao Shirai, assistant public relations director at Sumitomo, tells questioners that what the company is doing here is not really "shoulder-taping." The workers are merely choosing on their own to follow the company's guildelines, which specify the not tapping on the shoulder," he says, "if they leave of their own will."

The resisting workers insist this is a distinction without a difference. Once shown the guidelines, the workers in those older age brackets know they are expected to leave and fear other pressures if they do not they contend.

"It is very humiliating and it is very hard to stay around any longer" once the worker knows what is expected of him, says the Sohyo federation's Shoji. "An atmosphere is created in which it is very painful to stay around."