Japanese consumers, who have seen their currency more than double in value against the dollar in the last three years, are immersed in an unprecedented frenzy of conspicuous consumption.

Millions of cash-rich Japanese are gobbling up everything from $300,000 "leisure condos" to $3,000 high-tech toilets; from custom Danish furniture to top-of-the-line German cars; and from $30-a-pound American beef to $100 boxes of Swiss chocolates - all in record numbers and amounts, according to recent marketing and consumer surveys.The reason is that despite their substantial disposable incomes, few Japanese workers can afford to buy the one thing they really want: a house.

In Tokyo, according to one bank survey, the average 800-square-foot dwelling sold for $719,000 in 1987. Even with an average income of more than $21,000 a year (compared with $18,000 in the U.S.), few Japanese can afford to buy homes in the overpriced and overpopulated cities.

So, like Toshihiko Kuwahata, 38, a computer software engineer who earns the equivalent of $55,000 a year, they load their cramped apartments with an array of electronic gadgetry, appliances and furniture, the range of which probably is unmatched anywhere in the world.

Kuwahata's modest four-room apartment in western Tokyo is stocked with the latest stereo equipment, including two compact disc players, a digital tape deck, a 300-watt tuner, six sense-surround speakers, a large-screen stereo television and two video recorders. In addition, he has two computers, a video camera and a satellite dish.

In the tiny kitchen, Kuwahata's wife, Harumi, must juggle appliances to find counter space for everything from the latest computerized micro-wave/convection oven to an automatic breadmaker.

"The appliances are nice, but I would give them all up if we could just buy a nice big house somewhere," she said.

For some Japanese, the answer has been to buy weekend and vacation condos in rural areas where land is less expensive. About 10,000 "leisure condos" have been sold this year, according to a buying trends survey by Dentsu Inc., Japan's largest advertising agency.

These Japanese, says the survey, are breaking the old pattern of sticking every yen into low-interest savings accounts in the hopes of someday purchasing a house. Part of that is attributable to a recent change in the tax laws that no longer exempt savings accounts from taxation.

But another reason for the spending spree is the frustration among young Japanese who simply see no hope of ever being able to buy a house.

"When they say the average price of a house in Tokyo is $719,000, that isn't quite accurate," said Peter Kek, a real estate investment counselor. "In fact, to buy a house and lot anywhere within 1 1/2 hours of central Tokyo, you are looking at more than $1 million."

For that reason and because the government has begun to push domestic consumption rather than export earnings as the main engine of growth for the nation's economy, consumers are being coaxed into spending patterns that would have been unthinkable even five years ago.

"If you thought Americans were conspicious consumers, you haven't seen anything until you see the Japanese in action," said Ken Miyano, an American marketing consultant. "They are buying $80,000 German cars - and paying cash for them."

Tales of Japanese showing up on strangers' doorsteps in Hawaii and California and offering to pay cash on the spot for their houses are not apocryphal, said Miyano, who comes from Hawaii.

"The amount of disposable income in Japan is mind boggling," he said.

A whole industry has developed around the Japanese bathroom. The Toto Co.'s Washlet, for example, is a toilet with a padded and heated seat that sprays the user's posterior with a precisely aimed jet of warm water and then dries it with a blast of warm air. Toto is getting ready to market another high-tech toilet that will analyze urine and report the user's blood pressure and pulse rate.

"It says something about a society when people are spending $15,000 to $20,000 to turn their bathrooms into high-tech havens with automatic toilets, computer-operated saunas and o-furos (Japanese bathtubs)," said Miyano.

All of this, said Dentsu, is part of a "self-realization" process among the hard-working Japanese, who finally are discovering the "good life."

Maid services are flourishing in Tokyo, and the market for domestics is projected to reach $2.4 billion by 1995, according to another study. Imports of furniture hit a record $810 million during the first nine months of 1988, and consumers can choose from 100 brands of imported beers from 30 countries and 456 brands of wine from 20 countries.

Every week, Japanese homeowners are besieged with pamphlets and newspapers offering land, homes and condominiums in places such as California, Arizona, Australia's Gold Coast, Canada, Spain and Brazil.

"Buy Now While the Yen Is Mighty and the World Is at Your Feet!" blared one real estate brochure. "The World Is Yours, So Why Don't You Enjoy It?" asked another.

"Because of the trend to liberalize Japan's trade barriers, the Japanese consumer is discovering the rest of the planet and at the same time he is discovering that he can own much of what it makes too," said Kek, the real estate counselor. "The question seems to be whether or not that will satisfy Japanese consumers in the long run."