The present invention provides a novel radiosensitizeror anti-cancer chemotherapy sensitizer. In particular, theinvention provides a radiosensitizer or anti-cancer chemotherapysensitizer that can relieve the irritation of an affected areacaused by hydrogen peroxide, is safe when injected into a humanbody, and can delay or reduce the degradation of hydrogenperoxide and thereby can efficiently exert a radiationsensitizing effect and an anti-cancer chemotherapy sensitizingeffect. The radiosensitizer or anti-cancer chemotherapysensitizer comprises a combination of (a) hydrogen peroxide and(b) hyaluronic acid or salt thereof.
