PURPOSE: To join ring-like meltable substance smoothly and firmly to both surface of a metal ring by spot welding so as not to come off the metal ring by applying a tension to the ring-like meltable substance when jointing them.

CONSTITUTION: Torches 15 to be used for spot welding of four corners are provide on both layers 13 of meltable substance put on both sides of a spacer 12 with solder 14 with their tips obliquely outward so that they are wholly inclined toward the outside of the meltable substance 13, and further the inclination of each torch 15 from the vertical line is set to fall within a range of angle greater than zero degree and equal to or less than 30 degrees. By pressing the tips of these torches 15 against the corner sections of the meltable substance 13 to join the substance to the spacer at the same time, a tension can be aplied to the meltable substance 13 in the longitudinal direction, and hence the substance 13 can be protected from coming off a metal ring 12.