Provided are a novel method for treating cancer and an immunostimulant, which show a remarkably excellent antitumor effect and have few side effects. An antitumor agent and an immunostimulant, which are characterized by the combined administration of an azabicyclic compound and an immune checkpoint molecule modulator.顕著に優れた抗腫瘍効果を示し、副作用の少ない新規な癌治療方法及び免疫賦活化剤の提供。 アザ二環式化合物と免疫チェックポイント分子調節剤を併用投与することを特徴とする抗腫瘍剤及び免疫賦活化剤。