PURPOSE: To omit a step of mounting a seal rubber and provide a sheathed glow plug excellent in economy and seal properties, by fitting the seal rubber over a part of a center electrode which part is located on the direct upper side of an opening part of a sheathed heater, before fitting the sheathed heater into a main fitting.

CONSTITUTION: An annular seal rubber 4 is inserted from the other end side of a tube 7 of a sheathed heater 3, and is fixed on the direct upper side of an opening part 6 of the sheathed heater 3. A main fitting 2 is press fitted over the sheathed heater 3 in this condition, from the direction of an arrow. Simultaneously, a center electrode 5 with the seal rubber 4 mounted thereon is fitted into a central part of the main fitting 2. Thus, the seal rubber 4 is pressed into position, with deformation. As a result, good seal can be secured between the main fitting 2 and the center electrode 5 by resilience of the seal rubber 4, and the tuber 7 is pressed against the inner wall of the main fitting 2. Thereafter, a top opening part of the main fitting 2 is closed with an insulating stopper 11, which is fastened by a nut 12. An extra step for only fitting the seal rubber can be omitted.