A cap assembly is provided. The cap assembly comprises a main gasket surrounding the outer circumference of a safety vent and an auxiliary gasket surrounding the outer circumference of a current interrupt device (CID). The main gasket is supported by the auxiliary gasket, leaving no space between the main gasket and the safety vent. The cap assembly is constructed such that the auxiliary gasket prevents the main gasket seated on a beading portion from sagging. This construction makes the cap assembly effective in preventing an electrolyte from leaking. Further provided is a cylindrical secondary battery comprising the cap assembly.