PURPOSE: To directly drive a heating element by a commercial power source, by employing a heating element 5 with a positive resistance temperature coefficient for an electric heater to heat a resistor for gas sensing.

CONSTITUTION: A thermistor with a positive resistance temperature coefficient is connected as heating element 5 on an electric heater through a resistor 3 for gas sensing and an insulation layer 15. Electrodes 2 and 2 are provided at both ends of the resistor 3, and electrodes 4 and 4 at both ends of the heating element 5. A series circuit having a resistor 6 connected to the resistor 3 for gas sensing and the heat generating body 5 are connected to a commercial AC power source of voltage 100V. As the voltage of the AC power source 7 rises, the temperature of the heating element 5 should go up. But being a thermistor with a positive resistance temperature coefficient, the heating element 5 is suppressed in the power consumption with an increase in the resistance value and as a power source voltage lowers, conversely, the resistance value decreases to enhance the power consumption, which combine to keep the heating value of the heating element 5 constant. Even when ambient temperature such as room temperature changes, the resistance value of the heating element 5 is varied to adjust the power consumption thereby maintaining the temperature of the heat generating body 5 at a constant level.