Embodiments of the present disclosure relate to a cylindrical secondary battery that facilitates gas discharge by causing a bent shape of a cap plate to be deformed and broken if a temperature of a cell increases, such as due to an event. In one or more embodiments, a secondary battery includes: an electrode assembly; a case including a top end open to accommodate the electrode assembly; and a cap plate coupled to the top end of the case, and the cap plate includes a central region, an edge region, a bent portion between the central region and the edge region, and a vent portion including a notch configured to be broken by a certain pressure.