Disclosed is a potentiator of radiation therapy, which can reduce the dose ofradiation and can also reduce adverse side-effects when used in combinationwith cancer radiation therapy. The potentiator comprises an uracil derivativerepresented by the general formula (1) or a pharmaceutically acceptable saltthereof as an active ingredient. (1) wherein R representsa halogen atom or a cyano group； and R represents a 4- to 8-memberedheterocyclic group having 1 to 3 nitrogen atoms which may have a lower alkyl group,an imino group, a hydroxyl group, a hydroxymethyl group, a methanesulfonyloxygroup or an amino group as a substituent, an amidinothio group in which a hydrogenatom attached to a nitrogen atom may be substituted by a lower alkyl group, a guanidinogroup in which a hydrogen atom attached to a nitrogen atom may be substituted bya lower alkyl group or a cyano group, a lower alkylamidino group, or a 1-pyrrolidinylmethylgroup.
