It's Not All Downhill for Would-Be Japanese Skiers. WORKING HARD AT HAVING FUN

EVERY Friday night during winter, hundreds of young Japanese head from their downtown offices to Tokyo's major bus terminals. They travel all night, arriving early in the morning at a ski resort, leaping from the bus to the ski lifts. On Sunday afternoon, they board their coaches back to Tokyo, arriving in time to grab some sleep before dragging themselves into the office. But there is relief ahead for Japan's busy would-be leisure set. Come 1991 they will be able to hit the slopes, day or night, winter or summer, only 30 minutes from downtown Tokyo.

This September, the Mitsui Real Estate Co. will start constructing the world's first completely air-conditioned indoor ski mountain in Funabashi City, on Tokyo's outskirts. The ski slope - 1,600 feet long and 230-330 feet wide - will be covered with artificial powdered snow all year around.

``When people get a moderately satisfactory job and living conditions, the next thing is a pastime,'' says Satoru Adachi, assistant manager of the firm's public relations department. ``Plus, people want to do something good for their health, something that drives their stress away.''

In this nation of workacholics, leisure has finally become an accepted goal, even a nationally sanctioned one. The government is urging business and workers to move to the five-day work week (six days is common), take longer vacations, and consume more. In mountainous Japan, skiing has become a major fad for those, particularly the young, who are seriously pursuing a good time. The ski population reached 12.3 million in 1987, compared to 8.7 million in 1979, according to the Leisure Research Center.

``The companies that give a five-day workweek have been increasing and Japanese people have become affluent. Especially young people are aggressively spending money on leisure,'' says Kosho Yamada, a researcher at the center. ``Skiing itself has, compared to before, lost its image as a tough sport. It has come to have a more recreational image, such as going to a disco afterward.'' (Hotels at some Japanese ski resorts have discotheques and indoor swimming pools.)

Mitsui Real Estate predicts that 1.5 million people will come to the ``ski dome'' annually. The firm is targeting students and young office workers as its prime customers.

In this country where most office workers take holidays at the same time, just going to a ski resort takes a lot of work. Hotel reservations must be made more than three weeks ahead during the peak season. And traffic is heavy. Skiers who leave Tokyo by car on Saturday morning, hoping to reach the ski area by noon, often end up arriving in the evening.

So Mitsui's planners ``thought about how they themselves could go skiing,'' says Mr. Adachi, who admits he hasn't made it to his favorite slopes lately. The ski dome will rent ski wear and equipment to those who come right after work. It will also limit the number of skiers on the slope to 2,000 to avoid congestion. People waiting to ski will be able to amuse themselves at an indoor swimming pool in the same building or in adjacent shopping malls.

``If we just think about economic efficiency, we can earn billions of yen by building office buildings and apartment houses,'' says Adachi. ``What's unusual with this project is that overworking Japanese ... are doing business for the sake of their own recreation.''