Provides are a porous separator that prevents a short-circuit between two electrodes by using a porous nanofiber web where nanofibers have a core-shell structure, to thereby promote safety and thinning simultaneously. The porous separator includes: a porous nonwoven fabric playing a support role and having micropores; and a porous nanofiber web that is laminated on one side of the porous nonwoven fabric, and plays a role of an adhesive layer and an ion-containing layer when the porous nanofiber web is in close contact with an opposed electrode, wherein a portion of the porous nanofiber web is incorporated in a surface layer of the porous nonwoven fabric, to thus partially block pores of the porous nonwoven fabric and to thereby lower porosity of the porous nonwoven fabric. The porous nanofiber web has nanofibers obtained by spinning a mixture of a swellable polymer and a non-swellable polymer to have a core-shell structure.