To solve such problems that a sheath heater is thermally expanded by high-temperature exhaust gas, the temperature detection section of a sheath thermocouple is separated from the sheath heater, thus accurate temperature measurement is made infeasible in a heating element unit installed with the sheath heater in contact with the flow of the exhaust gas nearly at a right angle on the inside face of a can body to detect the temperature of the heater with the sheath thermocouple.

A fitting plate 4 is fixed to the inside face of the metal can body 1, and the sheath heater 2 is installed on the plate 4 in contact with the flow of the exhaust gas in the can body 1 nearly at a right angle. A U-shaped holding metal fixture 10 is fixed between the fitting plate 4 and the sheath heater 2, one side of the holding metal fixture 10 is bent inward for a spring property, and a through hole 11 is provided on the bent side. The tip of the sheath thermocouple 7 detecting the temperature of the sheath heater 2 is inserted from the through hole 11 of the holding metal fixture 10, and its temperature detection section 9 is brought into contact with the sheath heater 2.