A para-oriented polyamide porous film comprising a fibril having a diameter of not more than 1.mu., with fibrils planarly arranged as a network or nonwoven fabric and laminated in a layer, the thermal linear expansion coefficient at 200.degree.-300.degree. C. within .+-.50.times.10.sup.-6 /.degree.C. and 30-95% vacant spaces. Also, a battery separator using the porous film. Also, a production process of: (a) forming a film-like material from a solution containing 1-10% of a para-oriented aromatic polyamide having an inherent viscosity of 1.0-2.8 dl/g and 1-10% of a chloride of an alkali metal or an alkali earth metal in a polar amide or polar urea solvent; (b) maintaining the film-like material at not less than 20.degree. C. and not more than -5.degree. C. to deposit the para-oriented aromatic polyamide; and (c) immersing the film-like material in an aqueous or alcoholic solution to elute the sovlent and chloride of the alkali metal or alkali earth metal, then drying to obtain the para-oriented aromatic polyamide porous film. The film has uniformity and fine vacant spaces, which cannot be accomplished by a nonwoven fabric. Characteristics of a para-aramid (e.g., high heat resistance, high rigidity, high strength, etc.) are used.