A secondary battery includes a can having an opening at a side thereof, an electrode assembly inside the can, the electrode assembly including a first electrode, a second electrode, and a separator disposed between the first electrode and the second electrode, a one-unit cap covering the opening of the can, the one-unit cap including a current interrupt unit, a cap-up on the current interrupt unit, a positive temperature coefficient (PTC) device between the current interrupt unit and the cap-up, and an adhesive unit attaching the current interrupt unit to the cap-up to integrate the current interrupt unit, the PTC device, and the cap-up with each other, and an insulating gasket sealing a space between the can and the one-unit cap to seal the can.