A rectangular secondary battery includes a current breaking mechanism with improved rigidity against vibration and impact. The current breaking mechanism is interposed in the middle of a current path electrically connected between an external terminal and a wound electrode group to break the current path by the rise of battery internal pressure. The current breaking mechanism includes a connection electrode arranged inside a battery lid and electrically connected to the external terminal, and a conductive plate bonded to the connection electrode and deformed by the rise of the battery internal pressure. The connection electrode includes a tabular member having a through hole in communication with the outside of the battery opening on a planar portion. The conductive plate includes a dome-shaped diaphragm portion gradually reducing its diameter as shifting in an axial direction and a flange portion extending radially outward from an outer peripheral portion of the diaphragm portion.