A porous membrane contains a polyethylene resin, in a core layer, pores of sizes that are relatively larger than those of pores in each of skin layers on the opposite sides are distributed, and the skin layers on the opposite sides have substantially same pore characteristics. A method for manufacturing a porous membrane includes the steps of: obtaining a mixture of a liquid-type paraffin oil and a solid-type paraffin wax; adding the mixture to a polyethylene resin to obtain a raw material resin mixture; extruding and cooling the raw material resin mixture; stretching the raw material resin mixture; and immersing the stretched raw material resin mixture in an organic solvent to extract a mixture of the oil and the wax.