A separator for a power storage device, wherein the separator is characterized by comprising: a polyolefin multilayer microporous film comprising polyethylene and polypropylene, the polypropylene concentration at the outermost sections being higher than the polypropylene concentration at the innermost section, and the polyethylene concentration at the outermost sections being lower than the polyethylene concentration at the innermost section, wherein the outermost sections of the polyolefin multilayer microporous film are the cross-sectional portions of the two outer films, when the multi-layer microporous film has been divided into 3 portions of equal thickness in the direction of lamination, and the innermost section is the cross-sectional portion of the film on the inside, and an active layer laminated on at least one side of the microporous film, wherein the active layer contains inorganic particles and at least one fluorine-containing vinyl compound selected from polyvinylidene-hexafluoropropylene fluoride (PVdF-HFP) and polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-CTFE), wherein the polyolefin multilayer microporous film is laminated in the order of a first microporous layer, a second microporous layer and a first microporous layer, wherein each first microporous layer contains polyethylene and polypropylene, wherein the second microporous layer contains polyethylene, wherein the proportion of polypropylene with respect to 100 weight% as the total polyolefin resin in the first microporous layer is 1 to 50 weight%, and wherein the ratio of the weight-average molecular weights of the polyethylene and polypropylene in the first microporous layer adhering to the active layer determined as described in the description and calculated using the formula MwE weight-average molecular weight of polyethylene / MwP weight-average molecular weight of polypropylene is 0.6 to 1.5. a polyolefin multilayer microporous film comprising polyethylene and polypropylene, the polypropylene concentration at the outermost sections being higher than the polypropylene concentration at the innermost section, and the polyethylene concentration at the outermost sections being lower than the polyethylene concentration at the innermost section, wherein the outermost sections of the polyolefin multilayer microporous film are the cross-sectional portions of the two outer films, when the multi-layer microporous film has been divided into 3 portions of equal thickness in the direction of lamination, and the innermost section is the cross-sectional portion of the film on the inside, and an active layer laminated on at least one side of the microporous film, wherein the active layer contains inorganic particles and at least one fluorine-containing vinyl compound selected from polyvinylidene-hexafluoropropylene fluoride (PVdF-HFP) and polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-CTFE), wherein the polyolefin multilayer microporous film is laminated in the order of a first microporous layer, a second microporous layer and a first microporous layer, wherein each first microporous layer contains polyethylene and polypropylene, wherein the second microporous layer contains polyethylene, wherein the proportion of polypropylene with respect to 100 weight% as the total polyolefin resin in the first microporous layer is 1 to 50 weight%, and wherein the ratio of the weight-average molecular weights of the polyethylene and polypropylene in the first microporous layer adhering to the active layer determined as described in the description and calculated using the formula MwE weight-average molecular weight of polyethylene / MwP weight-average molecular weight of polypropylene is 0.6 to 1.5. wherein the active layer contains inorganic particles and at least one fluorine-containing vinyl compound selected from polyvinylidene-hexafluoropropylene fluoride (PVdF-HFP) and polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-CTFE), wherein the polyolefin multilayer microporous film is laminated in the order of a first microporous layer, a second microporous layer and a first microporous layer, wherein each first microporous layer contains polyethylene and polypropylene, wherein the second microporous layer contains polyethylene, wherein the proportion of polypropylene with respect to 100 weight% as the total polyolefin resin in the first microporous layer is 1 to 50 weight%, and wherein the ratio of the weight-average molecular weights of the polyethylene and polypropylene in the first microporous layer adhering to the active layer determined as described in the description and calculated using the formula MwE weight-average molecular weight of polyethylene / MwP weight-average molecular weight of polypropylene is 0.6 to 1.5.