PURPOSE: To improve the hydrophilic property of the surface of a film and carry out impregnation of the film with an electrolytic liquid by exposing a part of a filler by carrying out a surface treatment for the surface of a synthetic resin film in which the filler of a hydrophilic inorganic material is dispersed.

CONSTITUTION: SiO2 as an inorganic hydrophilic filler is mixed with a polyolefine-based resin and dispersed by adding a dispersing agent. The resulting mixed material is extended by a biaxial extending method to prepare a finely porous film. When the synthetic resin film is treated by plasma discharging in decreased pressure atmosphere, the polyolefine resin covering the filler in the film surface is dispersed and damaged and thus a part of the filler is exposed. Owing to the surface treatment, the improvement of the surface shape in micro-level together with implantation of a hydrophilic functional group and an effect of making the filler hydrop hilic are totally achieved, so that high hydrophilic property is obtained. Using the film for a separator, an electrolytic liquid is impregnated within a short time.