A secondary battery which can prevent leakage of an electrolytic solution by improving the sealing property of an electrolytic solution injection hole formed at a cap plate comprises: an electrode assembly including a cathode plate, a separator and an anode plate; a can having an upper opening for receiving the electrode assembly; and a cap assembly which is combined with the upper opening of the can, and which includes a cap plate provided with an electrolytic solution injection hole at one side thereof. A groove is formed in an axial direction on an inner circumferential surface of the electrolytic solution injection hole. Thus, the contact area between a stopper and the electrolytic solution injection hole is increased, thereby improving the sealing property of the electrolytic solution injection hole.