Prosecutors on Thursday demanded 17 years in prison for the man who attacked a pop idol he was obsessed with in Koganei, western Tokyo, after she spurned his gifts and social media messages.

"It was an extremely self-centered act," the prosecutors said at the Tachikawa branch of the Tokyo District Court.

The victim attended the session and asked that the court to impose severe punishment on 28-year-old Tomohiro Iwazaki, who attacked her with a knife after confronting the idol about a gift she sent back to him before a performance in the city.