To maintain an oxygen concentration detector at a specified temperature through the control of electric energization of a heater by determining the temperature of the heater accurately even when an electrical resistivity changes.

Pads 18 and 19 comprising a conductive first metal material are arranged on the surface of a heater 16 and leads 25 and 26 comprising second metal material different in type from the first metal material are kept being pressed in contact with the pads 18 and 19. A voltage detector 29 is connected to the leads 25 and 26 and furthermore, a power source 27 is connected to the leads 25 and 26 through an opening/closed means 28. The power of the power source 27 is supplied to the heater 16 by a closed path of the opening/closing means 28 while an electromotive force generated at contact parts 44 and 45 between the pads 18 and 19 and the leads 25 and 26 is measured by the voltage detector 29 as factor corresponding to the temperature of the heater 16 in the closed path of the opening/closing means 28. The results of the measurement are reflected on the control of the electric energization of the heater 16.