A secondary battery having an improved electrical connection structure of a secondary protective device by positioning a thermo-breaker in a protective device groove, in order to improve the stability of the secondary battery while minimizing the occupation of the inner space of the can. The secondary battery includes an electrode assembly having positive and negative electrode tabs. A can contains the electrode assembly therein. A cap assembly has an insulated electrode terminal and a cap plate for sealing the top opening of the can. A protective device groove is formed on the bottom surface of the cap plate. A thermo-breaker is seated in the protective device groove while being connected between the first electrode tab and the electrode terminal. When the temperature inside the battery reaches a predetermined value, the bimetal loses contact and the electrical current inside the battery is interrupted, avoiding overcharge/over-discharge or explosion of the battery.