Japan, one of the industrialized world's great redoubts of male supremacy, has officially sworn off discrimination against its 24 million working women.

Last month, the Diet, or parliament, approved the country's first comprehensive law on equality between men and women in the workplace, following a lengthy debate between business, women's groups and legislators.

On paper, it seemed a radical step. Female workers have played a crucial role in building Japan's postwar prosperity but only recently have begun rising above the rank of office helper or assembly-line hireling.

But the bill has been condemned by many women's groups and liberal opposition parties as too watered down to have any effect. And many of its proponents concede that old practices regarding women's rights will not die quickly.

Even as the bill was before the Diet, a Tokyo country club barred Vice Foreign Minister Mayumi Moriyama, one of two women holding a cabinet-level job, from a Saturday golf meet sponsored by her ministry for foreign diplomats. Club rules ban women on weekends, club officials explained.

By some measures, modern Japanese women have been moving steadily toward equality. There were 40,000 women managers in 1970; by 1983, official statistics show, there were 120,000. Women anchor national news broadcasts and teach in the best universities. At traditional festivals, they have joined in the once all-male task of carrying portable Shinto shrines in procession.

Unlike the United States, Japan already has an equality provision in its constitution, inserted, ironically, by American occupation authorities after World War II. Over the years, Japanese courts have struck down some overtly discriminatory job practices.

Still, equality of the sexes remains alien to the average Japanese person. The new law is not the result of any sudden consensus for change but because the conservative and mostly male members of the ruling Liberal Democatic Party wanted to uphold Japan's reputation abroad by ratifying the United Nations Convention to Eliminate All Forms of Discrimination Against Women.

Traditionally, Japanese women stayed home. Two of the most common words for wife, kanai and okusan, derive from words that formerly meant "person inside the house." Since World War II, however, Japanese women have broken that rule in such numbers that they now make up 40 percent of the work force.

Still, "a lot of women want to get home by 5 p.m.," says Yukiko Fukagawa, a woman researcher at the Japan External Trade Organization.

The typical Japanese female worker takes a job after graduating from school, quits to marry and bear children, then returns to the labor market after her children are of school age. But now more women are staying on the job after marriage.

Despite their numbers, women in Japanese offices have two main jobs: serving tea and running errands for men. Dressed in company uniforms, they are called by the English initials "O.L.," for "Office Ladies."

Foreign firms seeking contracts here are sometimes advised not to include women in their negotiating teams; it will only make the all-male Japanese side uneasy, the firms are told.

Japanese factories routinely categorize jobs by sex. At an electronics plant of the Sony Corp., a journalist recently remarked that no men were working the assembly line. Women were "better suited" to this type of job than men, the manager explained.

Women figure heavily in the ranks of Japan's huge but lowly paid part-time work force. These people are hired and fired as the times require, allowing companies a cushion for costs that helps guarantee job security for the life-time staffs, which are dominated by men.

With less seniority, skill and time on the job, women also end up with lower wages. In 1983, the average weekly wage for a woman was about $170, less than half of what men were making, government figures show.

Japanese personnel offices make no secret that they are reluctant to train or promote women. Many companies, in fact, will not even accept applications from women seeking responsible jobs.

"The trouble is that women quit jobs so easily. You can never tell how long they will stay," said Hiroshi Kitamura, director of the personnel management division for the Japanese Federation of Employers Associations. "It's a big loss for the company if we spend a huge amount of money educating women and they quit right afterward."

But some women's groups are not sure the new bill will improve the situation. On the issue of hiring and promotion practices, the bill provides only that employers "should endeavor" to treat men and women equally.

"This is unique to Japan, to make a vague request for efforts," complains Mitsuko Yamaguchi, head of the Fusae Ishikawa Memorial Association, a women's rights group.

But Kimie Iwata, who deals with women's issues in the Ministry of Labor, said companies cannot be expected to alter personnel practices that have served them well.

"We thought it is not proper for the government to force employers . . . . Legislation should be based on the social and economic conditions of each country," she said.

The bill also amends previous laws that restricted overtime, evening working hours and certain "dangerous" occupations for women. These measures worked against equality, the bill's authors said. They also prevented women from putting in the long hours required for advancement in Japanese companies.

But some labor unions and women's groups don't see longer hours as a something women should strive for. "People are working too hard," said Yamaguchi, "men included." She contends that men should be working shorter hours, rather than women longer ones.

Yamaguchi said that despite the law's shortcomings, her organization welcomed it. "We had to ratify the convention," she said. "We can use it as a stepping stone for the year 2000 and beyond."