A non-aqueous secondary battery separator composed of a porous film made of an organic polymer, which includes a network-like support, swells in the electrolyte solution and retains the electrolyte solution, wherein the network-like support has a mean film thickness of 10-30 µm, a basis weight of 6-20 g/m, a Gurley value of no greater than 10 sec/100 cc, a McMullin number (25°C) of no greater than 10 and a (McMullin number x mean film thickness (µm)) product of no greater than 200 µm, and the separator has a mean film thickness of 10-35 µm, a basis weight of 10-25 g/mand a Gurley value of no greater than 60 sec/100 cc, or exceeding 60 sec/100 cc and no greater than 500 sec/100 cc. Both battery characteristics and safety are achieved by establishing a specific electrochemical relationship between the effective active substance content of the battery system and the overcharge-preventing function characteristic values.
