To produce a fluorine-containing polymer membrane type ion exchange membrane having excellent oxidation resistance and wide range ion exchange capacity, and suitable for a fuel cell inexpensively in a large amount.

This method for radiation-treating the polymer membrane by irradiating it with an ionizing radiation for crosslinking the fluorine-containing polymer membrane substrate material in the absence of oxygen at a temperature around its crystal-melting point and without progressing thermal decomposition is provided by using heat-generating bodies divided to a multiple heat-generating body elements to heat at a uniform temperature, also forcefully cooling the fluorine-containing polymer membrane substrate material partially for confronting the temperature elevation by the irradiation within an irradiated field of the ionizing radiation so as to be able to irradiate the fluorine-containing polymer membrane substrate material having a large surface area with the ionizing radiation over the whole positions in the irradiated field and also over the whole time of the treatment in a state of keeping always within an optimally defined temperature range.