Child pornographies in Japan are becoming problmetic recently and the regulation on child pornographic images has been discussed over and over by Japanese government. It is against the Japanese law to sell or share pornographic pictures of those children who are under 18 years old; however, it is not against the Japanese law just to obtain those chid pornograhies as personal entertainment.
According to Yahoo News Japan on January 7th, the file sharing site (such as CABOS) is one of the most common resources to get child pornographies and the number of people who were caught for using the file sharing site to share child pornographic pictures was twice as much as last year.
However, there are still many people who have not been caught for uploading the child pornographic pictures through websites and the cases have been increasing. Japanese government has been trying to regulate child pornographies more strictly recently; however it always conflicts with people's human freedom, including freedom of speech or fleedom of sharing information.
For example, in 2009, the government of Tokyo tried to regulate child pornographic manga, and tried to protect children under 18 years old by giving human rights to those fictional characters in manga who are under 18 years old. This would allow to prohibit using those fictional children characters in pornographic manga although they are not real persons. The purpose of this regulation was protecting those real children because it was assumed that those people who enjoy child pornographic manga also are very likely to purchase or share real children pornographies. However, this idea of regulation was withdrawn because it was considered to be the violation towards freedom of speech for those manga artists. The definition of obscenity in the regulation was very vague, and it would have harmed those manga artists who are actually not violating children's human rights.
I personally think this is interesting topic to follow since there are many different aspects to look at. For now, it is not against the Japanese law just to obtain children's pornographies; however, as long as there is demand, I do not think the supply of these children's sexual images will ever stop although it can be harmful for those children. However, at the same time, the freedom of speech as artists or freedom of enjoying entertainment are as much as important in our daily lives, and the Japanese government would need to face this complexity while regulating child pornographies.