The Tokyo city government is to ban sales to children of a manga comic that depicts incestuous relationships, the first time expanded rules on sexual content have been invoked.
Little Sisters Paradise! 2, published last month, will be classified as an "unhealthy publication" that must be kept out of children's reach.
The comic, a spin-off from an adult-oriented computer game with the same title, says on its cover: "More naughty days of a brother and five sisters."
A panel of experts for the Tokyo Metropolitan Government "has reached the decision that (this manga) meets the criteria. We are moving to publicise the decision" formally on Friday, an official in charge of youth affairs said yesterday.
The ruling only affects stores in Tokyo but does not bar them from stocking the title, which will remain freely accessible to those aged 18 or older in adult sections of bookshops.
Three years ago, Tokyo tightened an ordinance stopping children buying publications that "significantly stimulate sexual feeling".
Little Sisters Paradise! 2 is the first publication to fall foul of the 2011 amendment that expanded the rules to cover pictures or text that "glorifies" incest.
Kadokawa, the major Japanese publishing house that released the comic, declined to comment.
Japan's attitude to sexually explicit material often comes in for criticism. Possessing pornography involving children is not illegal but its creation and distribution has recently been criminalised.
The UN Children's Fund (Unicef) has long pressed Japan to tighten its child porn rules, which it says exacerbate a global problem already made worse by the internet.
Manga images, while they might be mere drawings, could be considered pornographic under internationally shared norms, said Hiromasa Nakai, Unicef's Japanese spokesman.
"Many people agree that there are horrible manga images out there and they need to be dealt with," he said.
The move to tighten rules on sales of explicit material to children came partially in response to criticism from foreign campaign groups. But it faced resistance at home from manga artists, free-speech advocates and publishers, who said it would impinge on freedom of expression.