He's accused of having sex with a minor (i.e. someone who's not considered to be a legal adult). The girl he had sex with is under the age of 20, which is the age in Japan where you are legally considered an adult. Plus, as was said previously, the girl has now gone to the cops and accused him of raping her.
To be honest, I feel worse for his partner, who's career is now, unfortunately, royally screwed because of what's happened.