When Morinobu Tomita retires from his $30,000-a-year job designing water pumps for hydroelectric power plants next July, the president of his company will bow deeply, handing him a silver sake cup and a check for an extra year's salary.

After 30 years on the job, he also expects to get a government-sponsored pension each month equal to about 60 percent of his regular pay along with $200 in company benefits.

That all sounds good on paper, he said, "but you really wonder whether it will be enough to make ends meet." For Tomita, who is 59, and millions of his aging counterparts in the workforce here, the cost-of-living spiral and government talk of whittling down Japan's costly social security system have tarnished the postwar Japanese dream of a financially worry-free retirement.

Step into any office complex, factory or retail shop in Tokyo's dozen or so burgeoning business districts and you will hear Japanese middle-level managers, blue-collar workers and owners of small businesses voice the same kinds of concerns about retirement security that are voiced by many Americans these days.

In recent years, Japan's senior citizen population has exploded, rapidly reducing the ratio between Japanese in the labor force and those receiving pension benefits from the government.

While many foreigners remain in awe of Japan's scintillating economic performance, Japanese officials and social analysts now worry that the country's rapid aging process will pick up speed through the end of this century and potentially prove a serious drain on the nation's future financial resources and its industrial vitality.

In the United States, a similar demographic shift has helped push the federal pension program to the verge of bankruptcy. To be sure, the problem is not nearly as acute in Japan, where booming economic growth, traditionally high rates of personal savings and companies' paternalistic employment practices have so far cushioned government-controlled pension reserves.

But Japan's population is now growing older, faster than those of other advanced industrial nations, and, by the year 2000, the number of retired persons is expected to double, accounting for nearly one-quarter of all Japanese.

"This will create an increasingly heavy burden on the young people entering the work force today who will have to support their elders," said Nagahisa Hiraishi, of the Japanese Social Development Research Institute.

Japan's largest state-run retirement insurance scheme, which covers major private companies and their workers, got started in 1941, only a few years after the Social Security system went into effect in the United States. Because of Japan's more stringent eligibility requirements, however, the serious crunch has not come here yet, said one Health and Welfare Ministry official.

The Employes' Pension Fund, like Social Security in the United States, pays inflation-indexed benefits according to the length of time and the amount a worker has contributed. But, in contrast to the 10 years of payroll payments required to qualify an individual for standard benefits in the United States, the average Japanese worker must pay into the plan for 20 years to collect minimum benefits and 30 years to get the maximum.

According to government figures, there are now 25 million employes contributing to the system -- one of eight similar government-controlled pension plans here -- and only 2 million retirees collecting benefits.

To build up reserves in anticipation of the onslaught it expects in coming years, the government has recently raised pension premiums in stages to 11 percent of workers' salaries, shared equally by employers and employes up to a ceiling of about $21,000 of yearly earnings.

But Health and Welfare Ministry officials predict that, if present trends of lower birthrates and longer life spans continue, the standard deduction could triple by the end of the century.

In paying its pensioners, the government adds a direct subsidy of 20 to 30 percent to these salary contributions from its annual budgetary appropriations. But, burdened with a yearly budget deficit expected to reach $55 billion in 1982, Tokyo has zeroed in on the pensions, free medical services and welfare benefits it provides its senior citizens.

While President Reagan has, at least momentarily, backed away from most Social Security cuts, Prime Minister Zenko Suzuki unveiled a plan before the Japanese Diet, or parliament Sept. 28 that would reduce government contributions to the national pension fund by 25 percent over a three-year period beginning in 1983. Details of the cutbacks are now being worked out in committee.

The proposal, which is central to a larger package of spending reforms outlined under Tokyo's new tight-fisted economic policy, so far has touched off no massive public outcry.

It has, however, stirred up concern among leaders in Japan's powerful business community that Tokyo's failure to guarantee retirement security for the growing numbers of aging workers who have helped fuel Japan's economic prosperity could create a potentially bitter backlash in this orderly society of 117 million. It could, the critics charge, upset the relatively cozy relations between management, workers and government here that have helped keep labor disputes to the minimum and productivity rates high.

In Japan, companies normally force their workers to retire at 55. To ease the drain on its resources, the government is attempting to get companies to keep workers on to at least 65. But in Japan's rigidly hierarchical system of lifetime employment, in which workers' wages generally rise according to the number of years served with the company, most employers have flatly rejected the idea because of the increased labor costs involved.

For workers such as Morinobu Tomita, the prospect of impending retirement raises unsettling questions in a society where traditional systems of family support have undergone rapid transformation since the war.

"In the old days," Tomita said, "people didn't expect much government help and put their expectations on their children. But now they have learned that it's necessary to watch out for themselves. Personally, I don't want to be a burden on my kids."

The average Japanese still saves 20 percent of his income, compared to only 5 percent in the United States, and Tomita has salted away a modest nest egg, which he feels he may be forced to rely on to augment his government pension.

Tomita, however, echoing the sentiments of many loyal company men his age, has retirement worries beyond those of living on a fixed income. "I won't know what do with myself after I retire. I have no hobbies and have worked so hard for so many years that I haven't spread my energies to anything else. I would like to stay with the company for as long as possible."