PURPOSE: To prevent the nut cracking by pressing a nut screwed to a center electrode by means of a split die caulking jig centripetally from the whole peripheral side, in fixing the center electrode to the metal fittings for mounting a glow plug.

CONSTITUTION: An elastic insulator 18 is sealed between a center electrode 1 and mounting metal fittings 10 accommodating and retaining a heat generator and a nut 2 is fixed to the screw part 5 of the center electrode 1 through an insulating member 11. In this case, in fixing the nut 2, for example, a ring nut 3 is used as the nut 2, the ring nut 3 is uniformly caulked in the direction arrow X-X over the entire peripheral side by caulking jigs 8 and 8. Upon this occasion, when caulking is carried out for the intermediate part 9 between upper and lower surfaces of the nut 3, the intermediate part 9 is pressed against and caulked to the screw part 5, and its outer peripheral surface is shaped into a hexagonal form.