reporter: as a school girl, remon katayama was also groped on public transit. she kept silent, out of shame, and because this form of sexual assault seemed to be an accepted part of japanese society. even today, it's dismissed as a trivial offense. victims and bystanders are encouraged to call the police, but that rarely happens. remon: even when you say something, the people around you often just act annoyed. they rarely help you. so, many have basically just quit saying anything. reporter: she recently developed "chikan radar," an app that allows users to report harassment and tag the location. it already has 40,000 registered users. remon: my app shows you where gropers are, and how many incidents have happened where. it's constantly updated. reporter: the app serves as a kind of warning system for other users. it won't solve the problem of chikan, but remon katayama hopes it will assure users, don't be ashamed, you're not alone, even if the police are no help. remon: the feeling of powerlessness has a lot to do with the fact that groping is seen as a normal