To provide a radiation therapy potentiator, which, when employed in combination with cancer radiation therapy, can reduce radiation dose and can mitigate adverse effects.The invention provides a radiation therapy potentiator containing, as an effective ingredient, a uracil derivative represented by formula (1) (wherein R1 represents a halogen atom or a cyano group; and R2 represents a 4- to 8-membered heterocyclic group having 1 to 3 nitrogen atoms and optionally having as a substituent a lower alkyl group, an imino group, a hydroxyl group, a hydroxymethyl group, a methanesulfonyloxy group, or an amino group; an amidinothio group in which a hydrogen atom attached to a nitrogen atom may be substituted by a lower alkyl group; a guanidino group in which a hydrogen atom attached to a nitrogen atom may be substituted by a lower alkyl group or a cyano group; a lower alkylamidino group; or a 1-pyrrolidinylmethyl group) or a pharmaceutically acceptable salt thereof.