A safety device for a battery which can prevent explosion of the battery due to overcharging or shortcircuiting. In the safety device, a current flows through generating unit, lead, terminal of a conductor case, PTC plate, annular terminal, contact to electrode lid. A disk spring, generally called a bimetal, is adapted to bend backward when it is heated to a predetermined temperature. If the generating unit heats up by producing gas due to overcharging or shortcircuiting, the disk spring is heated. When heated to a predetermined temperature, it will bend backward, while pushing up a moving piece, which in turn pushing up the contact, thus separating the contact from the annular terminal. The current-flow path in the battery is thus cut, so that the generating unit will not heat up any further.