To safely retain a soluble alloy piece 1 against soldering heat transmitted through lead conductors even in soldering with a short distance between the both lead conductors 2, 2 through restraint of early movement of the soluble alloy piece 1 due to its own heat radiation by setting its length L at 3.7 times its outer diameter d or less much lower than a conventional one, in a cylindrical case type alloy version thermal fuse.

Of the cylindrical case type alloy thermal fuse, the distance L between tips of the both lead conductors 2, 2 is to be 3.7 times or less of the outer diameter d of the soluble alloy piece 1.