The present invention provides a resin composition comprising the following resin (a) and filler particles. The use of this composition makes it possible to obtain a separator having excellent heat resistance. Resin (a): a polymer which comprises a structural unit represented by a formula (1), and which has a weight average molecular weight of 200000 to 500000, provided that the polymer does not comprise a structural unit represented by a formula (2),
wherein, Mn+ represents a metal ion, and n represents the valence thereof,