Tokyo police on Friday asked four Internet site operators to delete so-called ranking sites that could guide Web surfers to child pornography.
The four operators, based in Tokyo and Kanagawa and Hyogo prefectures, manage 14 Web sites that rank child pornography sites according to hit counts.
The ranking sites do not show images but can guide viewers to about 130 sites, thereby promoting the spread of child pornography, the police said.
Those Web sites show some 14,500 unexpurgated images of children believed to be 18 or younger, and another 14,300 images suspected of being child pornography.
The Metropolitan Police Department said it plans to establish criminal cases against the operators of the child porn sites based on the 1999 law against child prostitution and child pornography.
Earlier in April, the National Police Agency stepped up efforts to tackle child pornography after cases in Japan hit a record high in 2009.