A separator for a power storage device comprising: a polyolefin multilayer microporous film containing polyethylene and polypropylene, and an active layer disposed on at least one side of the polyolefin multilayer microporous film, wherein the polyolefin multilayer microporous film has a polypropylene concentration at the outermost sections that is higher than the polypropylene concentration at the innermost section, and a polyethylene concentration at the outermost sections that is lower than the polyethylene concentration at the innermost section, and wherein the active layer contains an acrylic-based resin and inorganic particles, and the weight ratio of the acrylic-based resin and the inorganic particles (acrylic-based resin/inorganic particles) in the active layer is 10/90 to 50/50, wherein the outermost sections comprise a first microporous layer, and the innermost section comprises a second microporous layer, wherein the first microporous layer comprises a first polyolefin resin comprising polyethylene and polypropylene, and the amount of polyethylene contained in the first polyolefin resin is 50 weight% to 99 weight%, based on 100 weight% of the total weight of the first polyolefin resin, and the amount of polypropylene contained in the first polyolefin resin is 1 weight% to 50 weight%, based on 100 weight% of the total weight of the first polyolefin resin, and wherein the second microporous layer comprises a second polyolefin resin comprising polyethylene, the amount of polyethylene contained in the second polyolefin resin is 50 weight% to 100 weight%, based on 100 weight% of the total weight of the second polyolefin resin, and the amount of polypropylene contained in the second polyolefin resin is 0 weight% to 25 weight%, based on 100 weight% of the total weight of the second polyolefin resin. a polyolefin multilayer microporous film containing polyethylene and polypropylene, and an active layer disposed on at least one side of the polyolefin multilayer microporous film, wherein the polyolefin multilayer microporous film has a polypropylene concentration at the outermost sections that is higher than the polypropylene concentration at the innermost section, and a polyethylene concentration at the outermost sections that is lower than the polyethylene concentration at the innermost section, and wherein the active layer contains an acrylic-based resin and inorganic particles, and the weight ratio of the acrylic-based resin and the inorganic particles (acrylic-based resin/inorganic particles) in the active layer is 10/90 to 50/50, wherein the outermost sections comprise a first microporous layer, and the innermost section comprises a second microporous layer, wherein the first microporous layer comprises a first polyolefin resin comprising polyethylene and polypropylene, and the amount of polyethylene contained in the first polyolefin resin is 50 weight% to 99 weight%, based on 100 weight% of the total weight of the first polyolefin resin, and the amount of polypropylene contained in the first polyolefin resin is 1 weight% to 50 weight%, based on 100 weight% of the total weight of the first polyolefin resin, and wherein the second microporous layer comprises a second polyolefin resin comprising polyethylene, the amount of polyethylene contained in the second polyolefin resin is 50 weight% to 100 weight%, based on 100 weight% of the total weight of the second polyolefin resin, and the amount of polypropylene contained in the second polyolefin resin is 0 weight% to 25 weight%, based on 100 weight% of the total weight of the second polyolefin resin. <EOS>