TOKYO -- With characteristic efficiency and ingenuity, the Japanese have brought the full force of modern technology to bear on a timeless human endeavor: the secret afternoon tryst.

All over the country, a special breed of hotels exists just for couples who want a quick fling in complete confidentiality. These establishments use state-of-the-art automation, permitting guests to pick a room, check in, eat and drink, enjoy themselves, pay the bill and depart -- all without seeing, or being seen by, another human being.

According to hotel industry estimates, Japan has more than 20,000 such places, known here as "love hotels," "fashion hotels" or "abeku hotels" -- the Japanese pronunciation of the French word "avec," or "with." Generally renting rooms for two hours at a time, the hotels are so popular that many have to provide waiting areas where guests can bide their time until a vacancy is available. In a teeming country where most people live in tiny homes or apartments jammed sardine-style among countless others, the love hotels flourish because they offer two precious commodities -- space and privacy.

Although intended primarily to accommodate illicit affairs, the love hotels also draw business from married couples and innocent lovers with nothing to hide.

"Oh, yeah, we get married couples in here; sometimes a couple of grandparents come in together," said Mamoru Saga, proprietor of a Tokyo love hotel called the San Eight. "You know, their house is so small, and the kids are running around, and the bedroom door is shoji {rice paper framed in wood}. It's just too noisy for them at home. They can't be alone."

Saga's hotel also gets its share of secretive couples, he said. "College kids come in; guys come in a lot with their secretaries. Sometimes they even do work in there."

Like many other love hotels, the San Eight has its own "frequent flyer" card, permitting discounted rates after a certain number of visits. For privacy reasons, the cards are issued by number, and no names are ever recorded. "That makes them really popular," Saga said. "Except a guy can get in trouble if his wife finds it in his pocket."

Love hotels are generally easy to locate in Japan. They tend to be clustered in particular neighborhoods or resort towns. They often take foreign names, either French ("C'est La Vie," "Chez Nous," "Petit Elegance") or English ("Paper Moon," "Dixy Inn"), and often the name is meant to be suggestive ("Hotel Passion," "Once More," "Sweet Lovers Inn," "Let's Come").

Another way to spot a love hotel is from the characteristic high wall that blocks the entrance so customers can slip in without being seen from the street. Many of the inns also have underground parking lots. In some cases, the parking places are equipped with a shield that comes down automatically behind the car so that its license plate can't be photographed.

When patrons enter a love hotel, sensing devices perceive their presence and trigger a tape recording that offers instructions on how to check in. Generally, the hotels have a lighted board in the tiny lobby displaying pictures of the various types of rooms and beds available. Some of the newer hotels offer computerized laser-vision displays to help guests choose the right room.

Having made a choice, the guests press a button next to the picture of the desired room. Signs then guide the couple to a desk, where they put money or a frequent-visitor card into a slot and obtain the room key.

In the room, there's generally a mini-bar offering snacks and drinks, with payment on the honor system. Many love hotels also sell a variety of potions promising to fortify strength, ardor, or both. Various types of condoms and "adult toys" may also be for sale. Guests can automatically order movies -- adult or otherwise -- to be shown on the room television set.

When the allotted time is up, a discrete bell rings in the room, telling the guests it is time to leave. The departure times for each room are often synchronized so that no two couples are in the halls or the elevator at the same time. Generally, the exit is through a back door so guests won't inadvertently run into somebody who happens to be arriving by the main entrance.

Rates vary, depending on the location and level of grandeur of the hotel. Here in Tokyo, the two-hour charge -- listed euphemistically on the hotels' signs as the "rest" rate -- runs roughly from $30 to $90. Most love hotels also offer an all-night fare -- listed as the "stay" rate and good from 10 p.m. to 10 a.m. -- for about 20 percent more.

Although love hotels are devoted to maintaining privacy, the method of providing total secrecy is not always met automatically anymore. In some places, the law requires that at least one desk clerk be on hand at all times. Some hotels deal with that by putting a shade in front of the clerk so he can't see the customers who check in.

Nowadays, moreover, the love hotels are attracting people who don't seem to care whether anybody sees them or not.

"It used to be, people came in here with their head down and went to the room as discreetly as possible," said Saga, manager of the San Eight. "But these college kids, they come in with a big smile and no embarrassment. Sometimes we even get two or three couples going up on the elevator at the same time."

Sure enough, as Saga was chatting in his hotel's narrow lobby one recent morning, a couple who looked to be about college age walked in calmly, inquired about rates, took a key and strolled off to a room.

"Kids today -- can you believe it?" an incredulous Saga exclaimed. "They come into a love hotel, and they have no shame about it at all."