A safety device for use in a secondary battery having a battery case formed with a hole open to outside of the battery case, and a generating unit airtightly housed in the battery case. The safety device includes a disk spring mounted on the battery case to close the hole and having one side thereof convexed toward the inside of the battery case. A switch is provided on the backside of the disk spring. It is adapted to change over under a stress applied from the disk spring when the disk spring bends backward so that the other side thereof is convexed toward the outside of the battery case. The switch is provided in a current flow path of the secondary battery.