The present invention disconnects a shorted current path and shorts an open current path in a prescribed order and irreversibly switches a current path. A switch element (26) for energizing a second heating element (11) is connected to the second heating element (11). An open terminal of a first heating element (2) is connected to a connecting terminal (16) for the second heating element (11) and an open-side soluble conductor (12). As a result of the operation of the switch element (26), the second heating element (11) is energized and generates heat, the open-side soluble conductor (12) fuses, and a main circuit (25) and second circuit (24) are disconnected. As a result of the the fusing of the open-side soluble conductor (12), the first heating element (2) is energized and generates heat, a short-side soluble conductor (3) fuses, a switch (4) is shorted, and current flows between the main circuit (25) and a first circuit (23).