PURPOSE: To prevent destruction of an insulating tube at the time of action of a thermal fuse of soluble alloy type.

CONSTITUTION: Lead wires 1, 2 are connected to a soluble alloy 3 to insert a connecting unit of the lead wire and the soluble alloy into an insulating tube 5. In a thermal fuse of sealing between both end opening parts of the insulating tube and the lead wires with insulating substances 6, 7, when assumed, for instance, (d) (mm) for distance between a surface of the soluble alloy and an internal surface of the insulating tube and I (A) for rated current at AC250(V) rated voltage, the fuse is formed so as to obtain a relation where d≥0.06I+0.2.