A microporous polyolefin membrane made of a polyethylene resin as a main component, and having (a) a shutdown temperature of 135° C. or lower, at which the air permeability measured while heating at a temperature-elevating speed of 5° C./minute reaches 1×105 sec/100 cm3, (b) an air permeability change ratio of 1×104 sec/100 cm3/° C. or more, which is a gradient of a curve representing the dependency of the above air permeability on a temperature at the air permeability of 1×104 sec/100 cm3, and (c) a meltdown temperature of 150° C. or higher, at which the air permeability measured while further heating after reaching the above shutdown temperature becomes 1×105 sec/100 cm3 again.