The maddening numbers game the 55-year-old college dropout unleashed on an unsuspecting planet now appears in more than 600 newspapers, on thousands of websites and in dozens of books in about 70 countries. Unsurprisingly, then, most visitors to Nikoli, the Tokyo firm Kaji named after the winner of the Irish 2,000 Guineas in 1980, want to know one thing: what will he do for an encore? "I don't know, " he says. "We had no idea sudoku would be so successful.

It depends on what our writers come up with."

We're sitting in what some have dubbed the puzzle palace, a narrow building in abusiness district of Tokyo. Below us, in an atmosphere resembling a convention of chess players, a small, earnest team edits and refines the 1,000 puzzles that flood into the company every month. Among the handful that will be added to the company's roster and road-tested in this puzzle-mad country could lurk the new sudoku.

It is from here that recent successes first emerged.

The creative engine-room, however, is mostly elsewhere, in bedrooms, kitchens and commuter trains across Japan. Nikoli's puzzle writers, who design 90% of the games and work for a flat fee, are drawn from a cross-section of ordinary Japanese life:

teenagers, housewives, salarymen and pensioners.

Two-thirds of them are high school and university students; the oldest is an 81-year-old retired teacher.

What they have in common, Kaji says, is inventiveness, a willingness to "serve" others, and time . . . lots of time. "Students like to sit around and dream up games, " he says. Some develop a couple of hit puzzles, then don suits and ties and disappear for ever into company life. Kaji says he feels their pain. In the early 1970s, his life appeared mapped out when he began studying literature at Keio University, one of Japan's most prestigious private colleges and a passport to corporate success. But he found it "boring", and began gambling and playing tennis.

As for why sudoku was widely embraced, he says its compactness helped. The man who launched it, however, has seen very little of the loot it has generated worldwide. Kaji did not patent his discovery, but seems unfazed by that. "More than the money, it is gratifying to see the explosive worldwide growth of our puzzles. That makes me very happy."