A secondary battery including a cap assembly with a structure improved for tight sealing and a plug therefor are provided. The secondary battery includes a battery unit formed by rolling a stack of a negative plate, a separator, and a positive plate, a cap assembly, and an electrode tab drawn out from one of the negative plate and the positive plate and electrically connected to an electrode port from the cap assembly. The cap assembly includes: a cap plate having a port aperture and an electrolyte injection hole; the electrode port connected via the port aperture to the electrode tab; a gasket installed to surround the electrode port so as to insulate the electrode port from the cap plate; and a plug which is plugged into the electrolyte injection hole by pressing and whose upper rim matches an upper edge of the electrolyte injection hole. The plug has a fixed position with respect to the electrolyte injection hole, so that the battery may be more hermetically sealed, without causing failure in welding, and becomes more reliable.