Polyolefin Microporous Membrane, Separator for Electricity Storage Devices, and Electricity Storage Device

According to the present invention, a microporous membrane contains a polyolefin resin and inorganic particles; the primary particle diameter of the inorganic particles is 100 nm or less; the content of the inorganic particles is 10-60% by mass or 10% by mass or more but less than 40% by mass based on the mass of the microporous membrane; and the retention time at 150° C. is less than 200 seconds or the retention time at 145° C. is more than 1 second but less than 300 seconds in the thermal behavior evaluation of the microporous membrane.