A secondary battery includes an electrode assembly including: a winding having a positive electrode plate, a negative electrodes plate, and stacked together with a separator interposed between the positive and negative electrode plate; a can having an opening portion to receive the electrode assembly; a cap assembly including a cap plate to close the opening portion of the can, the cap plate having an electrolyte injection hole arranged therein; and a first extension groove having a greater diameter than that of the electrolyte injection hole, the first extension groove being arranged in an entering portion of the electrolyte injection hole and having at least one hollow portion arranged on the surface of the first extension groove.