In 1219, while visiting Tsurugaoka Hachiman-gu Shrine to attend the Seii Taishogun (literally, "great general who subdues the barbarians") of Kamakura bakufu (Japanese feudal government headed by a shogun), MINAMOTO no Sanetomo's inauguration as Minister of the Right, Sanemasa witnessed Sanetomo's assassination.
建保7年（1219年）、鎌倉幕府征夷大将軍源実朝の右大臣就任の鶴岡八幡宮参詣に随従してその暗殺を目にする事になる。