A method for manufacturing a separator, the method comprising: preparing a porous substrate having pores which is a polyethylene porous membrane; coating a slurry on at least one surface of the porous substrate, the slurry containing inorganic particles dispersed therein and a first binder polymer dissolved in a first solvent; coating a binder solution on the slurry, the binder solution containing a second binder polymer dissolved in a second solvent; and simultaneously drying the first and second solvents to form a porous polymer outer layer of the second binder polymer and an organic-inorganic composite inner layer, wherein the porous polymer outer layer has pores formed while the second solvent is dried, and the organic-inorganic composite inner layer has pores or interstitial volumes formed between the inorganic particles when the inorganic particles are bonded and fixed to each other by the first binder polymer while the first solvent is dried; the loading amount of the binder solution is adjusted such that a loading amount of the porous polymer outer layer made from the second binder polymer is in the range between 0.1 and 3.0 g/m 2; wherein the content of the second binder polymer in the binder solution is in the range between 0.1 and 30 weight%; wherein each of the first binder polymer and the second binder polymer is, independently, at least one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinylacetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan and carboxyl methyl cellulose, or their mixtures. preparing a porous substrate having pores which is a polyethylene porous membrane; coating a slurry on at least one surface of the porous substrate, the slurry containing inorganic particles dispersed therein and a first binder polymer dissolved in a first solvent; coating a binder solution on the slurry, the binder solution containing a second binder polymer dissolved in a second solvent; and simultaneously drying the first and second solvents to form a porous polymer outer layer of the second binder polymer and an organic-inorganic composite inner layer, wherein the porous polymer outer layer has pores formed while the second solvent is dried, and the organic-inorganic composite inner layer has pores or interstitial volumes formed between the inorganic particles when the inorganic particles are bonded and fixed to each other by the first binder polymer while the first solvent is dried; the loading amount of the binder solution is adjusted such that a loading amount of the porous polymer outer layer made from the second binder polymer is in the range between 0.1 and 3.0 g/m 2; wherein the content of the second binder polymer in the binder solution is in the range between 0.1 and 30 weight%; wherein each of the first binder polymer and the second binder polymer is, independently, at least one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinylacetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan and carboxyl methyl cellulose, or their mixtures.