A sheath heater for a glow plug comprises a sheath tube having a closed front end portion, and an open rear end portion. An insulating powder is charged into a gap between the sheath tube and a heat-generating resistor disposed within the sheath tube. A seal member includes an expanded portion extending radially outwardly from an outer circumference of the seal member, and a non-expanded portion of a smaller outside diameter than the expanded portion and formed on the outer circumference of the seal member at least at a leading end of the seal member. The seal member is fitted into the open rear end portion of the sheath tube such that the leading end enters the sheath tube first. The sheath tube is deformed radially inwardly around the seal member for sealing the heat-generating resistor and the insulating powder contained in the sheath tube.