There is provided a radiosensitizer containing an iron compound, more specifically, a radiosensitizer containing an iron compound selected from an inorganoiron compound, such as iron chloride, iron oxide, iron hydroxide, and iron sulfate; and an organoiron compound, such as saccharated ferric oxide, iron citrate, iron gluconate, chondroitin sulfate/iron colloid, cideferron, ferrotrenine, iron fumarate, iron pyrophosphate, a porphyrin-iron complex, and an albumin incorporating porphyrin-iron complex.