There is provided an agent for potentiating the effects of an anti-tumor agent.An anti-tumor effect potentiator containing, as an active ingredient, a uracil compound represented by the following formula (I) or a pharmaceutically acceptable salt thereof: wherein X represents a C1-5 alkylene group and one of methylene groups constituting the alkylene group is optionally substituted with an oxygen atom; R1 represents a hydrogen atom or a C1-6 alkyl group; R2 represents a hydrogen atom or a halogen atom; and R3 represents a C1-6 alkyl group, a C2-6 alkenyl group, a C3-6 cycloalkyl group, a (C3-6 cycloalkyl) C1-6 alkyl group, a halogeno-C1-6 alkyl group or a saturated heterocyclic group.