An antitumor effect potentiator of another antitumor agent, including an acylthiourea compound represented by formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient: wherein R 1 represents a C 1-6 alkyl group which may have a substituent, while the substituent represents any one of a hydroxyl group, a C 3-10 cycloalkyl group, a C 1-6 alkoxy group which may have a substituent, a C 1-6 alkylamino group which may have a substituent, a C 1-6 alkanoylamino group, a C 1-6 alkylsulfonyl group, a C 6-14 aromatic hydrocarbon group which may have a substituent, a saturated or unsaturated heterocyclic group which may have a substituent, a C 1-6 alkylaminocarbonyl group which may have a substituent, a saturated or unsaturated heterocyclic carbonyl group which may have a substituent R 2 represents a fluorine atom or a chlorine atom and R 3 represents a hydrogen atom, a fluorine atom, or a chlorine atom.
