Method for producing propylene-based resin microporous film and propylene-based resin microporous film

A method for producing a propylene-based resin microporous film, comprising:

• an extrusion step of melt-kneading a propylene-based resin at a temperature that is equal to or higher than a temperature higher than the melting point of the propylene-based resin by 20° C. and equal to or lower than a temperature higher than the melting point of the propylene-based resin by 100° C. in an extruder, and extruding the propylene-based resin through a T die attached to the tip of the extruder to obtain a propylene-based resin film;
• a first stretching step of uniaxially stretching the propylene-based resin film at a surface temperature of −20 to 100° C. and a total stretching ratio of 1.05 to 1.6 times;
• a second stretching step of repeating, a plurality of times, a stretching basic step in which the propylene-based resin film after the first stretching step is uniaxially stretched in the stretching direction of the propylene-based resin film at a surface temperature that is higher than the surface temperature of the propylene-based resin film in the first stretching step and equal to or lower than a temperature lower than the melting point of the propylene-based resin by 10 to 100° C., wherein the total stretching ratio is 1.05 to 3 times and between the successive stretching basic steps, the stretching ratio in the forward-side stretching basic step is adjusted so as to be lower than the stretching ratio in the backward-side stretching basic step;
• and
• an annealing step of annealing the propylene-based resin film after the second stretching step at a surface temperature that is equal to or higher than the surface temperature of the propylene-based resin film in the second stretching step and equal to or lower than a temperature lower than the melting point of the propylene-based resin by 10° C.