Disclosed is a novel radiation sensitizer or anti-cancer chemotherapy sensitizer. Specifically disclose is a radiation sensitizer or anti-cancer chemotherapy sensitizer which 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 so as to exert a radiation sensitizing effect or an anti-cancer chemotherapy sensitizing effect with efficiency. The radiation sensitizer or anti-cancer chemotherapy sensitizer comprises a combination of (a) hydrogen peroxide and (b) hyaluronic acid or a salt thereof.
