A polyolefin microporous membrane, the membrane having, when measured by DSC, a degree of crystallinity of from 65 to 85%, a lamellar crystal/crystal ratio of from 30 to 85%, a crystal length of From 5 nm to 50 nm and an amorphous length of from 3 nm to 30 nm, and a polyolefin microporous membrane, the membrane having, when measured by X-ray diffractometry, crystal size of from 12.5 nm to 13.5 nm and a degree of crystallinity of from 64 to 68%.
