An antitumor effect potentiator for other antitumor agents, which contains, as an active ingredient, an acylthiourea compound represented by general formula (I) or a pharmaceutically acceptable salt thereof. (In the formula, R1 represents a C1-6 alkyl group which may have a substituent, and the substituent is a hydroxyl group, a C3-10 cycloalkyl group, a C1-6 alkoxy group which may have a substituent, a C1-6 alkylamino group which may have a substituent, a C1-6 alkanoyl amino group, a C1-6 alkylsulfonyl group, a C6-14 aromatic hydrocarbon group which may have a substituent, a saturated or unsaturated heterocyclic group which may have a substituent, a C1-6 alkylamino carbonyl group which may have a substituent, or a saturated or unsaturated heterocyclic carbonyl group which may have a substituent； R2 represents a fluorine atom or a chlorine atom； and R3 represents a hydrogen atom, a fluorine atom or a chlorine atom.)本發明提供一種抗腫瘤劑效果增強劑，其係其他抗腫瘤劑之抗腫瘤效果增強劑，且以通式(I)所表示之醯基硫脲化合物或其藥學上所容許之鹽為有效成分：(式中，R1表示亦可具有取代基之C1-6烷基，作為該取代基，表示羥基、C3-10環烷基、亦可具有取代基之C1-6烷氧基、亦可具有取代基之C1-6烷基胺基、C1-6烷醯基胺基、C1-6烷基磺醯基、亦可具有取代基之C6-14芳香族烴基、亦可具有取代基之飽和或不飽和雜環基、亦可具有取代基之C1-6烷基胺基羰基、亦可具有取代基之飽和或不飽和雜環羰基中任一種，R2表示氟原子或氯原子，R3表示氫原子、氟原子、或氯原子)，
