The exact date cannot be found in "Sonpi Bunmyaku" (a text compiled in the 14th century that records the lineages of the aristocracy) and "Heiji Monogatari" (The Tale of Heiji), but there's a description that he was killed in 1180.
『尊卑分脈』『平治物語』月日は特定できないものの治承4年に殺害されているとの記載がある。