It's no secret that Japan's advertising market is one of the world's largest. Indeed, the world's biggest advertising firm is a Japanese one.

In 2006, the top 10 Japanese advertising agencies billed clients ¥6 trillion (37 billion euro), according to Dentsu, Japan's leading agency. Although the overall figure has been stagnant for the past few years, it still constitutes a more-than-significant sum.

What remains a secret for many, though, is exactly how their market operates.