An antitumor effect potentiator for potentiating one or more other antitumor agents, comprising, as an active ingredient, an imidazooxazine compound represented by Formula (I), or a pharmaceutically acceptable salt thereof, wherein A, B, C, and D represent C-R 1a , C-R 1b , C-R 1c , and C-R 1d , respectively, or one or two of A, B, C, and D represent an N atom； at least two of R 1a , R 1b , R 1c , and R 1d represent hydrogen, and the other(s) represent(s) halogen； cyano； C 1-6 alkyl that may have hydroxyl group(s) as substituent(s)； C 1-6 alkoxy； carbonyl having, as a substituent, hydroxyl, amino, optionally substituted mono- or di-(C 1-6 alkyl)amino, or mono- or di-(C 1-6 alkoxy)amino； or an unsaturated heterocyclic group； R 2 represents phenyl, pyridyl, or thienyl； R 3 represents hydrogen, methyl, ethyl, or cyclopropyl； and R 4 represents hydrogen or hydroxy.
