wherein it is heated at the melting temperature or a higher temperature under an inert gas atmosphere having an oxygen concentration of 10 torr or less within a range of an irradiation dose of from 1 kgy to 10 mgy an example of the ionizing radiation to be used includes u03b3 rays electron rays x rays neutron radiation and high energy ions etc in case of applying ionizing radiation it is preferable to heat the unmodified fluororesin molded article at its crystalline melting point or a higher temperature when a ptfe is used as a fluororesin material it is required to expose ionizing radiation to the unmodified fluororesin material which is under a heating condition at a higher temperature than 327 u00b0 c being the melting point of the fluororesin material in case of employing a pfa or an fep it is required to expose such a material to ionizing radiation under a heating condition wherein the former pfa is heated at its melting point of 310 u00b0 c or a higher temperature while the latter fep is heated at its melting point of 275 u00b0 c or a higher temperature to heat the fluororesin at its melting point or a higher temperature means to activate molecular motion of backbone chains which constitute the fluororesin whereby it becomes possible to efficiently accelerate cross linking reactions among molecules however excessive heating brings about adversely cutting and decomposition of the molecular backbone chains accordingly a heating temperature should be.