Voyeur(1992)

Nozokiya

Ko Higaonna is a happy college student, a trusting young man with an alluring girlfriend, Satomi. But her busy schedule leaves little time for their relationship, and thus few opportunities for Ko to explore his sensual desires. When Ko discovers that a voyeur is spying on them, he embarks on a quest to uncover what exactly Satomi is doing in her spare time - and finds that he must also confront his own repressed longings.

(Source: Viz Media)