To ensure soldering between a jointing lead-pipe and a cable lead sheath by vacuuming the inside of a jointing lead pipe for decompression, sucking molten solder between the jointing lead pipe and a cable lead sheath, and making a clearance between both of them disappear.

When one end of a cable lead sheath 3 is covered with a jointing lead pipe 4 previously before formation of a core joint part 2, band-shaped soldering layers 6, 7 are applied previously to the inner surface of a sleeve 4a in the vicinity of both ends of the jointing lead pipe 4 and the outer surface in the vicinity of the terminal of the cable lead sheath 3, and a vacuuming hole 4b is also formed. After formation of the core joint part 2, heat melting is conducted by bringing the soldering layer 6 on the jointing lead pipe 4 side into close contact with the soldering layer 7 at the cable lead sheath 3 side, padding a solder 5 between the end part of the jointing lead pipe 4 and the cable lead sheath 3. At the same time, by evacuating the inside of the jointing lead pipe 4, a part of the melted solder 5 is sucked in the clearance between the soldering layers 6, 7 so that it may be gone.