A secondary battery according to the present invention comprises: a battery case having an electrode assembly and an accommodation unit for accommodating an electrolyte; a fluid inlet/outlet port formed at the battery case and having a moving path, through which a fluid can move, so as to enable discharge of an inner gas and re-injection of the electrolyte; a gas discharge unit for restricting thedischarge of the internal gas which passes through the fluid inlet/outlet port; and an electrolyte re-injection unit provided so as to enable the re-injection of the electrolyte through the fluid inlet/outlet port.