PURPOSE: To sufficiently secure an amount of flux to an amount of a low melting point metal by specifying the ratio of the outer diameter of the low melting point that of a coating layer of the metal.

CONSTITUTION: An outer diameter R of an insulating layer 11 is √2 to 2.4 times the outer diameter rn of a soluble metal member 2 of a low melting point. A flux 3 is applied in a thickness a little larger than the outer diameter R of the insulating layer 11 and is inserted into an insulating cylinder 4. The amount of the flux 3 will be more than needed if the outer diameter R of the insulating layer 11 is more than 2.4 of the outer diameter r2 of the soluble metal member 2. This results in a too narrow gap between the insulating cylinder 4 and insulating layer 11 so that it is difficult to inject sealing agent into the gap. Division and formation into phere, of the soluble metal member 2 are therefore promoted when a thermal fuse operates in spite of carbonization of the flux 3 due to arc. Accordingly, quick operation of the thermal fuse can be enhanced.