PURPOSE: To obtain the hydrophilic film which has good workability and mechanical strength, its electric resistance is low and is cheap by a method wherein a sulfonic group is introduced to basic body consisting of a copolymer of olefin and vinyl acetate as the hydrophilic film.

CONSTITUTION: A sulfonic group is introduced to a copolymer film of olefin such as ethylene, etc. and vinyl acetate as the hydrophilic film used for electrolysis, electric dialysis, particularly, for a separator, etc. of a battery, by employing fuming sulphuric acid, sulfur trioxide, etc. The film obtained in such a manner is extremely cheap, is low in electric resistance, is strong in bending, is thin in thickness and has wide industrial applications.