A fine pore formation agent for a porous resin film is provided which comprises inorganic particles satisfying (a) 0.1≦D50≦1.5 (μm) (D50: average particle diameter of particles in 50% cumulative total by weight from the larger particle side by micro-track FRA), (b) Da≦20 (μm) (Da: maximum particle diameter by micro-track FRA), (c) 3≦Sw≦60 (m2/g) (Sw: BET specific surface area measured by nitrogen adsorption method), (d) Ir≧1.0×105 (Ω·cm) (Ir: volume resistivity (Ω·cm).The fine pore formation agent for a porous resin film is capable of providing a resin composition giving a porous resin film useful in uses for electric parts such as capacitors and battery separators.