The Fair Trade Commission has searched six publishers of children’s books over allegations they had formed a cartel, sources close to the case said Saturday.

The six companies include Tokyo-based major publishing house Gakken Co., the sources said.

The FTC searched some 10 locations, including the headquarters of the six firms, they said.

The publishers are suspected of fixing the prices and number of pages of books published every month for children under school age, the sources said.

Books published by the six companies are classified into two types depending on readers’ ages. Their publications for older children are priced at around 420 yen and have 40 pages.

These books are usually sold via kindergartens on annual contracts. The antitrust watchdog suspects the six publishers held a meeting every fall, when they advertise such sales contracts, the sources said.

Toshiro Komatsu, president of Gakken, insisted his company is in fierce competition with its rivals, citing the dwindling number of children in Japan. He confirmed that the publisher had been searched by the FTC but refrained from further commenting on the issue.

In Japan, total sales of children’s books amount to some 15 billion yen every year, with combined sales by the six companies accounting for about 75 percent of the total.