To provide a separator for battery capable of contributing to suppression in capability variation of a battery.SOLUTION: A method for producing a polyolefin microporous film includes: (a) an extrusion step of melt-kneading and extruding a resin composition containing a polyolefin-based resin and a pore-forming material; (b) a sheet molding step of discharging the kneaded resin into a sheet form from a mouthpiece, and cooling and solidifying the discharged resin; (c) a primary stretching step of the sheet-like molding obtained in the step (b) using a biaxial stretching machine for stretching the sheet-like molding in longitudinal and lateral directions; (d) a step of extracting the hole-forming material from the stretched sheet obtained in the step (c); and (e) a secondary stretching step of stretching the sheet obtained in the step (d) 1.1 times or more in at least a uniaxial direction. A stretching section in a path length of secondary stretching in the step (e) is 9% or more and 20% or less.SELECTED DRAWING: None