The present invention relates to a radiotherapy enhancer that can reduce the radiation dose and adverse drug reactions when used in combination with a cancer radiotherapy. There is provided a radiotherapy enhancer comprising, as an active ingredient, a pyridine derivative represented by general formula (1):wherein R1, R2, and R4 may be the same or different from one another and represent a hydrogen atom, hydroxy group, or protected hydroxy group, excluding the case where R1, R2, and R4 are all a hydrogen atom, and R3 represents a halogen atom, amino group, carboxyl group, carbamoyl group, cyano group, nitro group, alkyl group having 1 to 6 carbon atoms, alkenyl group having 2 to 6 carbon atoms, or carbonyl group containing an alkoxy group having 1 to 6 carbon atoms).
