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 R1 represents a C1-6 alkyl group which may have a substituent, while the substituent represents any one of 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 alkanoylamino 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 alkylaminocarbonyl group which may have a substituent, 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.
