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—R1a, C—R1b, C—R1c, and C—R1d, respectively, or one or two of A, B, C, and D represent an N atom at least two of R1a, R1b, R1c, and R1d represent hydrogen, and the other(s) represent(s) halogen cyano C1-6 alkyl that may have hydroxyl group(s) as substituent(s) C1-6 alkoxy carbonyl having, as a substituent, hydroxyl, amino, optionally substituted mono- or di-(C1-6 alkyl)amino, or mono- or di-(C1-6 alkoxy)amino or an unsaturated heterocyclic group R2 represents phenyl, pyridyl, or thienyl R3 represents hydrogen, methyl, ethyl, or cyclopropyl and R4 represents hydrogen or hydroxy.
