A separator containing a substrate and a release layer formed on at least one side of the substrate wherein the separator shows a strain of not more than 7% when a load of 5N/20 mm is applied for one minute in the pulling direction, and when the substrate is cut in half in the thickness direction to divide the separator into two, an apparent elastic modulus in the pulling direction of one of the divided separators is larger than an apparent elastic modulus in the pulling direction of the other divided separator.