Amid Tokyo's seedy love hotels, private detectives ply a lucrative trade on the trail of adulterous men

On a rainy Saturday night in the neon-drenched streets of Shinjuku, central Tokyo, Kenji Shimura looks like a thousand other salarymen: off-the-rack black suit, sensible shoes and a face made for anonymous middle management in one of the city's insurance firms. But few salarymen get paid to spend much of their working week prowling the area's love hotels armed with a notebook and a pocket video camera.

On a rainy Saturday night in the neon-drenched streets of Shinjuku, central Tokyo, Kenji Shimura looks like a thousand other salarymen: off-the-rack black suit, sensible shoes and a face made for anonymous middle management in one of the city's insurance firms. But few salarymen get paid to spend much of their working week prowling the area's love hotels armed with a notebook and a pocket video camera.

"Friday and Saturday nights are my busiest times," says the 38-year-old, who has been here so often he knows every nook and cranny of this gaudy square mile of hourly rate hotels. "That's when couples who are having affairs meet up and this is usually where they come. It's amazing to see people meeting to have sex and then go home to their families as though nothing has happened."

Mr Shimura's job as a private investigator for one of Japan's fastest-growing detective agencies is straightforward: catch his targets on video walking through the doors of one of these hotels with someone on their arm, and present the results to a waiting partner.

"You think they would be upset," he says. "But I've seen women cry with happiness because they know they can get out of their marriage. It makes me believe it's all worthwhile."

Mr Shimura has never been busier. Japan's growing divorce rate, which has climbed for the 13th consecutive year and doubled since 1975, has helped the organisation he works for, the Galu Detective Agency, grow from a tiny four-man operation in 1990 to an 800-strong nationwide franchise today.

The National Investigators Association of Japan (NIAJ) says that there are more than 10,000 detective agencies in a country where traditionally strong marriage ties are withering. Middle-class housewives are willing to shell out up to 200,000 yen a day (£1,000) to get the dirty on a troublesome spouse.

"Fooling around is a game for the well-off," says Takahisa Suenaga, Galu's manager, who sits in a low-lit office surrounded by the tools of his trade, including miniature cameras and a portable lie-detector machine. "Poor men are too busy working. What are they going to do, bring their mistresses to a stand-up noodle bar? Men always had affairs, but they have more opportunities today because of internet dating sites and because there are more available women. Japanese wives used to put up with it but now they're more likely to call us."

Mr Suenaga has been with the agency for seven years. "I was hired a few years back to tail a middle-aged man and I caught him going into a love hotel with his own 17-year-old daughter. They had been having sex for months. I can't tell you how hard it was to show that video to his wife." Even in cases like this, the agencies are under no obligation to contact the police, who seldom get involved in family affairs, part of the agencies' attraction to families that want discreet solutions to problems.

Corporate bankruptcies, rising debt and the sometimes hair-raising antics of the young are also helping to keep Japan's growing army of snoops in business. Mr Shimura also says he is being asked to follow more teenagers. "Mothers wonder where their daughters are getting money to go shopping for designer clothes. I followed a 16-year-old recently and discovered she had been moonlighting as a prostitute for a delivery-health business [a pizza-type service that delivers cut-price prostitutes to your door]."

A fifth of the detective work is locating missing people, often bosses and managers on the run from debt-ridden companies and loan sharks. Two million people are struggling with bad debts, and individual bankruptcies reached a record 214,600 in 2002. This has helped to spawn an underground world of loan sharks who advertise their wares, like many of the detective agencies, on utility poles. Mr Shimura said: "It's hard work finding people on the run from debt. I've found ex-bosses working as road labourers in Kyushu [in Japan's far south]."

Hiroshi Tahara, president of the National Investigators Association, says: "Japan has no national licensing system for private investigators.Anybody can walk in off the street and become one.There is a lot of fear and mistrust in society right now that is helping the industry grow." He is unhappy at the antics of a growing number of wakare-saseya, or relationship break-up specialists, who set honey traps to lure straying husbands away from lovers. The break-up specialists have also mutated into something nastier, says Mr Tahara: revenge specialists. "The object here is to lure the man away from his lover and break his heart. To leave him ruined."

Next year, the authorities are to introduce new legislation to rein in such activities. Most expect the result to be a proper licensing system and a shake-out of the more unethical practices.

The staff at Galu believe their agency will thrive. "This is a growing market and it is about to mature," says Mr Suenaga. "There are so many people like me heading into their thirties and middle-age. This is the age group that most often requests investigations into extramarital affairs. The economy is getting worse so there are more cases of missing people. Stalking and domestic violence is on the rise and the police do not want to get involved. There is no end to people's misery."