An antitumor effect potentiator for other antitumor agents, which contains an imidazooxazine compound represented by general formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient. (In the formula, each of A, B, C and D represents C-R1a, C-R1b, C-R1c or C-R1d, or alternatively one or two among A, B, C and D are substituted by N atom(s) at least two among R1a, R1b, R1c and R1d represent hydrogen atoms and the rest represents a halogen atom, a cyano group, a C1-6 alkyl group which may have a hydroxyl group as a substituent, a C1-6 alkoxy group, a carbonyl group which may have a hydroxyl group, an amino group, an optionally substituted mono- or di-(C1-6 alkyl)amino group or a mono- or di-(C1-6 alkoxy)amino group as a substituent, or an unsaturated heterocyclic ring R2 represents a phenyl group, a pyridyl group or a thienyl group R3 represents a hydrogen atom, a methyl group, an ethyl group or a cyclopropyl group and R4 represents a hydrogen atom or a hydroxy group.).一種抗腫瘤效果增強劑,係另一抗腫瘤劑之抗腫瘤效果增強劑,且以咪唑并化合物或其藥學上可接受之鹽作為有效成分,該咪唑并化合物係以下列通式(I)表示者:通式(I)(式中,A、B、C及D分別表示C-R1a、C-R1b、C-R1c、及C-R1d,或者表示前述A、B、C、D中之任一者或兩者經N原子取代;R1a、R1b、R1c、R1d中之至少二者表示氫原子,其他分別表示:鹵原子;氰基;可具有羥基作為取代基之C1-6烷基;C1-6烷氧基;具有羥基、胺基、可具有取代基之單或二(C1-6烷基)胺基或者單或二(C1-6烷氧基)胺基任一者作為取代基之羰基;或是不飽和雜環基;R2表示苯基、吡啶基或噻吩基;R3表示氫原子、甲基、乙基或環丙基;R4表示氫原子或羥基)。
