Patented is a microporous polyolefin membrane with fine fibrils. The membrane includes a polyolefin with a weight-average molecular weight of 5_105 or more or polyolefin fibrils with a weight-average molecular weight of 5_105 or more. The fibrils are connected to each other. The average pore size of the membrane is 0.05-5 mm and the crystal lamellas of the polyolefin are inclined at an angle of 80-100° to the plane constituting the membrane. The crystal lamellas account for at least 40% of the total lamellas both on a section cut in the mechanical direction and on a section cut in the direction perpendicular to the mechanical direction and in the thickness direction.