A separator for an electric double layer capacitor, characterized in that a thickness of the entire separator is 25 .mu.m or less, a layer of an ultrafine fibrous aggregate prepared by an electrostatic spinning process is contained, an average fiber diameter of ultrafine fibers constituting the ultrafine fibrous aggregate layer is 1 .mu.m or less, and a maximum pore size of the ultrafine fibrous aggregate is not more than 3 times a mean flow pore size is disclosed.