Their designers are as popular as rock stars and their fashion shows attract crowds as big as rock concerts. Stores are jammed with young women looking at and buying designer clothes, and on the streets women have latched onto every fashion trend around from Bermuda shorts to long culottes and even some mini-skirts.

The passion for fashion in Japan starts with the fact that there is a lot of money around to spend on clothes. "We are very rich," says Kazuko Okuma, who represents Clinique Laboratories Inc. in Japan. And, says Ichiko Koike, a writer and museum consultant: "Real estate costs are so high we can't afford bigger places to live so we spend our money on clothes. It is one of the few ways to spend money." Rock star Agnes Chan has another view: "It is all part of being mannerly and polite to dress well and to be well made up."

Last month one of the local newspapers decided to cash in on designer popularity by sponsoring a joint fashion show by two top Japanese designers, Issey Miyake and Kenzo. They sold tickets at $15 and $25 apiece and sold out 3,000 tickets for each of the three shows. An additional 3,000 people attended the fourth show held, like the others, in a huge ice arena.

Next week Kansai Yamamoto is putting on a show of his own, and already has sold 15,000 tickets. Price of admission? A $10 Kansai T-shirt, which must be worn, or at least carried, to get in.

Each Sunday there is a wildly spontaneous fashion show in Tokyo as thousands of teen-agers flock to the blocked-off streets near the Yoyogi Sports Center in Harajuku. Many arrive from Tokyo suburbs by subway, wearing their T-shirts and jeans or even their school uniforms and change into fancy get-ups in the public bathrooms. For the young girls, the clothes can be out of Arabian Nights, somewhat like our Halloween costumes; eastern exotica bridesmaids dresses; or American rock 'n' roll circa the 1950s. The guys seem to be mixing the image of James Dean with the black leather of London punks. Girls wear bows in their hair and lace short gloves. The guys are big on greasy-looking pompadours.

The kids dance in groups around huge transistor tape recorders. Last Sunday, the big music boxes and the shopping bags of clothes were protected by sheets of plastic from a drizzling rain that did not seem to bother the dancers. Some groups are extremely well-rehearsed and choreographed, others are just getting started with simple Lindy steps. Many keep the rhythm by blowing metallic whistles and clapping.

The dancing at Harajuku started about four years ago, apparently when discos began to curb young teen-agers' admission to the clubs. "My mother doesn't mind my coming here, but she hates discos," said a 14-year-old girl in a full-skirted pink polka-dot dress, puffed out with a stiff crinoline underneath. "My mother says coming here and dancing in the free air is healthy," giggled her friend, who was dressed almost identically.

The girls in this group, who wear navy jumpers and white blouses to school all week, bought their dresses for Harajuku at the Takenoko Boutique of Noritake Otake, currently the hottest boutique along a nearby crowded block of tiny shops. Many of the shops are the size of apartment-house closets and sell gadgets such as whistles and combs, colorful pins, Snoopy knickknacks, teen cosmetics and a few Deely Bobbers, the headband antenna with stars and balls on spring wires. And, of course, the clothes these kids wear are also in the shops here.

Teen-agers aren't the only Sunday shoppers in Tokyo. Parco II, a six-story building in the Shibuya part of Tokyo, with 55 tiny designer boutiques, is so crowded you have to wait to get on the escalator. Every make-up demonstrator at the first floor cosmetic counters has several customers and more waiting. Saturday and Sunday are normally big shopping days in Japan, but this is the time of Ochugen, a summertime gift-giving period which is almost as good for stores as Christmas. It helps that it coincides with the time when workers get their bonuses.

In the best boutiques like Issey Miyake and Yohji Yamamoto, the clothes are mostly one-size-fits-all. Pants are small, medium and large, an indication of their full and comfortable cut that seems to work well for more than just the tall and slim. The colors are usually dark and, quite often, several shades of black, such as a blackened green as well as plain black, plus dark blue and gray. "Indigo is the favorite color of the Japanese from ancient times," writes Ikka Tanaka and Kazuko Koike in a book on colors in Japan soon to be released in America. The authors believe the Japanese are very careful about color choices because they have emotional significance for them. Additionally, they point out that dark blue, which is almost a national color, has been around a long time in Japan. It has always been the favored color for workmen and fishermen. "Indigo dye had a fragrance when new and mellowed when old," they write in explaining its appeal.

American designers are featured in many of the department stores, and Ralph Lauren and Bill Blass are currently the front-runners. Blass expects to be in 15 stores by next fall, and he predicts doing $9 million in business on clothes alone. "Japan is the big future for me," Blass said recently.

A word about designer names in Japan. All names are used with the family name first and the given name second, so Miyake Issey and Yamamoto Kansai are the way names are promoted here. They are appropriately turned around on labels when sold in Washington.

Although you see few women wearing kimonos in Tokyo, the exhibit of kimonos by Itchiku Kubota, shown in a gallery on the top floor of the Seibu department store in its suburban branch in Finnibashi has attracted tens of thousands of visitors. Kubota has used a 16th century tie-dyeing technique to make silk kimonos in a modern way. He thinks that if women didn't have to wear the obi and could wear modern shoes, they might just wear kimonos again. But he is not expecting a sellout in spite of the admiriation of the viewers. His kimonos sell for $70,000 each; so far, he has sold 15.