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 C 1-5 alkylene group and one of methylene groups constituting the alkylene group is optionally substituted with an oxygen atom;
R 1 represents a hydrogen atom or a C 1-6 alkyl group; R 2 represents a hydrogen atom or a halogen atom; and R 3 represents a C 1-6 alkyl group, a C 2-6 alkenyl group, a C 3-6 cycloalkyl group, a (C 3-6 cycloalkyl) C 1-6 alkyl group, a halogeno-C 1-6 alkyl group or a saturated heterocyclic group.