Lay and professional judges sentenced a Chuo University graduate Thursday to 18 years in prison for stabbing his professor to death last year on the school's Korakuen campus in Bunkyo Ward, Tokyo.

Presiding Judge Yukihiko Imasaki said that while Ryuta Yamamoto's delusional disorder was taken into account, the court ruled he was of diminished capacity and could still be held accountable for his crime.

The judges held Yamamoto, 29, responsible for murdering Hajime Takakubo, 45, a professor in the the science and engineering department, on Jan. 14, 2009.

Yamamoto attacked Takakubo from behind in the men's room near his office, stabbing him 47 times with a 27.8-cm blade.

During the Tokyo District Court trial, Yamamoto said he murdered Takakubo because he believed a "pressure group" led by the professor was constantly watching him and trying to kill him in retaliation for damaging the professor's reputation.

While Yamamoto's disorder had a strong influence on his motive for killing Takakubo, his actions leading up to the murder, which were carefully planned, derived more from his personality, the court ruled.

Neither the prosecution nor the defense argued against Yamamoto's diminished capacity. Prosecutors had demanded a 20-year prison term.

Yamamoto "still seems to believe that this pressure group exists," Imasaki said. "But we think you're mistaken. Please continue to receive treatment during your prison term and after you have served your time."