1. An agent for enhancing the antitumor effect of another antitumor agent comprising an acylthiourea compound represented by the formula (I) or a salt thereof as an active ingredient: wherein R 1 is an optionally substituted C 1-6 alkyl group R 2 is a fluorine atom or a chlorine atom and R 3 is a hydrogen atom, a fluorine atom or a chlorine atom, and wherein the optional substituent is a member selected from the group consisting of a hydroxyl group, a C3-7 cycloalkyl group, optionally substituted olefinoxy group, optionally substituted C1-6 alkylamino group, C1-6 alkanoylamino group, C1-6 alkylsulfonyl group, optionally substituted Saromatic hydrocarbon group, optionally substituted saturated or unsaturated heterocyclic group, optionally substituted C1-6 alkylaminocarbonyl group, optionally substituted saturated or unsaturated heterocyclic carbonyl group. The agent for enhancing the antitumor effect of claim 1, wherein in the formula (I), R 1 represents a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group or a sec-butyl group, R 1 represents a fluorine atom or a chlorine atom, and R4 represents a hydrogen atom, a fluorine atom or a chlorine atom, wherein the alkyl group optionally contains as a substituent a hydroxyl group, a cyclohexyl group, a methoxy group, an ethoxy group, an isopropyloxy group, a diethylamino group, an acetylamino group, methylsulfonyl a phenyl group, a pyrrolidinyl group, a morpholino group, a dioxolane group, a tetrahydropyranyl group, a pyridyl group The agent for enhancing the antitumor effect of claim 1, wherein in the formula (I), R 1 represents a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group or a sec-butyl group, R 1 represents a fluorine atom or a chlorine atom, and R4 represents a hydrogen atom, a fluorine atom or a chlorine atom, wherein the alkyl group optionally contains as a substituent a hydroxyl group, a cyclohexyl group, a methoxy group, an ethoxy group,
