The present invention provides a novel radiosensitizer or anti-cancer chemotherapy sensitizer. In particular, the invention provides a radiosensitizer or anti-cancer chemotherapy sensitizer that can relieve the irritation of an affected area caused by hydrogen peroxide, is safe when injected into a human body, and can delay or reduce the degradation of hydrogen peroxide and thereby can efficiently exert a radiation sensitizing effect and an anti-cancer chemotherapy sensitizing effect. The radiosensitizer or anti-cancer chemotherapy sensitizer comprises a combination of (a) hydrogen peroxide and (b) hyaluronic acid or salt thereof.
