A cap assembly according to the present invention is a cap assembly for a secondary battery coupled to an open end of a battery can which does not have a beading portion, wherein the cap assembly comprises: a top cap protruding from the top thereof so as to form an anode terminal; a safety element arranged beneath the top cap such that the safety element contacts the top cap; a safety vent arranged to contact the safety element; a gasket which covers edges of the top cap, the safety element and the safety vent; a cover which covers the gasket to enable the top cap, the safety element and the safety vent to tightly contact together; and a member to be welded, which is arranged on the cover, and which has an outer end protruding further outwardly than the outer end of the cover such that the member to be welded is loaded at the open end of the battery can, wherein the portion of the member to be welded, protruding further outwardly than the outer end of the cover has an upper surface with a first notch such that the portion with the first notch is welded to the open end of the battery can.