It is a method for modifying a resin (6), the method for hydrophilizing the surface of the resin (6). High-concentration ozone gas and an unsaturated hydrocarbon gas are supplied to the surface of the resin (6), and the surface of the resin (6) is hydrophilized. The high-concentration ozone gas is generated by re-vaporizing liquid ozone obtained by liquefaction and fractional distillation of an ozone-containing gas. Ozone gas having an ozone concentration of 50 vol % or greater is used as the high-concentration ozone gas. A gas containing an unsaturated hydrocarbon with a carbon number of 10 or lower which has a double bond or a triple bond is used as the unsaturated hydrocarbon gas.