PURPOSE: The titled film having mechanical toughness, having pores with a specific average pore diameter, forming a large number of pores on the surface and section uniformly, communicating the pores with one another.

CONSTITUTION: For example, a cast film having a laminated structure with a crystallinity not less than 60%, contg. ≥30% crystals other than α type, having ≤0.1μ thickness of the laminated longer and a principal molecular chain in the longitudinal direction, having a difference between a refractive index of longitudinal direction and that of width direction not less than 0.012μ, is subjected to orientation and heat-treatment in such a way that not less than 10% degree of orientation based on total degree of orientation is carried out at not higher than 80°C, to give the desired film having not less than 80% pores with an average pore size 0.01W 0.08μ and a ratio of a crystal other than α type crystal to total crystal being 3W8%.

USE: A film for permeable package, separating film for battery, etc.