Japan faces a grim task that may prove more vexing than flooding the world with cars or cracking the computer chip market: having a good time.

Although older Japanese cling to the work ethic that rocketed this country from World War II devastation to economic superpower status, their children have decided all work and no play made Japan a dull nation.Shoko Shima, 31, works 45 hours a week as a translator, about average for the Japanese.

But she also takes time to work out regularly at a health club and play tennis and golf. "People used to work for work's sake or their family's sake. But now many of us work to have free time," Shima said.

"More and more young people are choosing part-time jobs so they can work when they want instead of being tied to a big company."

In some ways, Japan today is a nation bent on enjoying itself: the number of official holidays surpasses that of the United States, and young people's vision of the future does not include working grueling hours and blind loyalty to a company.

"Many people today say their hobby is work, and the older ones especially say it with a sense of pride," said Sumiko Iwao, a psychologist who teaches at Tokyo's Keio University. "But younger people today are saying `No, that's not my cup of tea.' "

"In the future, productivity will decline, but that's not bad because it is much better for Japanese to enjoy life and relax than just work all the time," Ms. Iwao said.

But the old ways continue, especially in the highly charged business world.

"If you want to succeed in business in Japan, you still have to work from 9 to 9," Shima said.

The average Japanese worked 2,150 hours in 1986 - about 225 hours more than Americans and almost 500 more than West Germans, according to Labor Ministry statistics. In 1987, the Japanese figure edged up by 18 hours.

The government embraced the idea of a five-day workweek in 1985, partly to reduce big trade surpluses. More leisure time, the theory goes, would equal increased spending on imports, an expansion of recreational industry, more domestic spending.

But no one said it would be easy.

Office worker Kayoko Kumagai, 41, divides her free time between household chores, bowling and working out at her health club. But her 37-year-old husband, a company worker, "just lies around the house" on his one day off a week, she said.

Like most Japanese, Kumagai has grown used to working six days a week with only Sundays off.

"Not many people realize they are working too much. They take it for granted," said Mrs. Kumagai. She counts her husband among them, and added, "My husband wouldn't be able to figure out what to do if he had weekends off."

When the government announced plans to give about 58 percent of public servants two Saturdays off a month, people complained they could no longer pay taxes, apply for travel visas or visit the national library every Saturday. The so-called "twin holidays" went into effect Jan. 14.

Financial institutions went to a five-day workweek Feb. 1, and the government hopes other businesses will follow suit. It has targeted 1993 for a uniform five-day workweek.