A polyolefin-based porous separator, including a first polyolefin-based porous film on a first surface of a second polyolefin-based porous film, and a third polyolefin-based porous film on a second surface of the second polyolefin-based porous film, each of the first and third polyolefin-based porous films containing inorganic particles having an average particle size of 10 nm to 100 nm, a thickness ratio of the first polyolefin-based porous film, the second polyolefin-based porous film, and the third polyolefin-based porous film being 0.5 to 1.5:1 to 6:0.5 to 1.5, and thermal shrinkage rates of the separator in a machine direction and a transverse direction measured after standing at 120° C. for 1 hour each being 5% or less, and air permeability of the separator being 250 sec/100 cc or less.