Abstract: Provided is an agent for alleviating side effect of antitumor drug, the agent alleviating side effect of antitumor drug when used in combination with the antitumor drug. Disclosed is an agent for alleviating side effect of antitumor drug, the agent containing a uracil compound represented by the following Formula (I) or a pharmacologically acceptable salt thereof as an active ingredient: wherein 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.