A method for manufacturing a separator includes (S1) preparing a porous substrate having pores, (S2) coating at least one surface of the porous substrate with a first solvent, (S3) coating the first solvent with a slurry containing inorganic particles dispersed therein and formed by dissolving a binder polymer in a second solvent, (S4) drying the first and second solvents simultaneously to form a porous organic-inorganic composite layer on the porous substrate. Since the phenomenon that the pores of the porous substrate are closing by the binder polymer is minimized, it is possible to prevent the resistance of the separator from increasing due to the formation of the porous organic-inorganic composite layer.