A thermal fuse includes an insulating case having a bottom and having an opening provided therein, a fusible alloy provided in the insulating case, a lead conductor having one end connected to the fusible alloy and other end led out from the insulating case through the opening of the insulating case, a flux provided on the fusible alloy, and a sealer for sealing the opening of the insulating case. The volume of a space between the fusible alloy in the insulating case and the sealer is larger than the volume of the flux. Sealing of the fuse is prevented from deteriorating, and the insulating film is prevented from damage even when the thermal fuse is used for breaking a large current at a high voltage.