To provide an electrolytic film precursor having a high modulus of elasticity and giving easily an ion exchange film of a low electric resistance by means of hydrolysis.

After film formation, this precursor is irradiated with electron beams in such conditions as within the temp. range above the glass transition point of the film and below the decomposing point and with an oxygen concentration of 5000 ppm or less, and then is subjected to a heat treatment for two minutes to one hour. The precursor manufactured by this process has an ion conductive functional group precursor as a fluorine-containing polymeride electrolytic film precursor, whose dynamic modulus of elasticity at 90°C is 4×1.05 thru 30×105 Pa. The electrolytic film is obtained by hydrolyzing the electrolytic film precursor.