TECHNOLOGY FOCUS - POLYMERS - Preferred Film: A porous base material is impregnated into a solution or dispersion liquid which contains a monomer convertible into a group which functions as electrolyte after polymerization. The base material is then interposed between a film. The solution or dispersion liquid, is excluded until the space between film and porous base material becomes 5 microns or less. The monomer is polymerized and electrolyte polymer is formed. The electrolyte polymer filled in the porous base material is contacted with solution of same or different electrolyte polymer. The porous base material comprises polyolefin or cross-linked polyolefin preferably cross-linked polyolefin. Preferred Property: The decreasing rate of proton conductivity (B) of film measured in the state which removed water or electrolyte liquid with respect to proton conductivity (A) measured in the state in which polymeric electrolyte film surface is adhered with water or electrolyte liquid, is 50% or less. The proton conductivity (A) and (B) is more than 1 S/cm2 or more. EXAMPLE - A cross-linked polyethylene film of thickness 16 microns, porosity 40% and average pore size 0.1 microns, was used as porous base material. The porous film was immersed in polymer precursor solution which consists of 2-acrylamide-2-methyl propane sulfonic acid (50 g), N,N'-methylene bis acrylamide (5 g), non-ionic surfactant (0.05 g), ultraviolet ray radical polymerization initiator (0.005 g) and water (50 g). The porous base material was taken out from the solution and interposed between two PET films of thickness 50 microns. The film was arranged on a flat glass plate and pressed and irradiated with ultraviolet ray. The polymer precursor was polymerized. A layer of polymer precursor was formed between base material and PET film on one side. The PET film was removed and the electrolyte film was obtained. The obtained film had proton conductivity of 27 S/cm2 and 16 S/cm2, in a state adhered with water and without water, respectively. The decreasing rate of proton conductivity was 41%.