PURPOSE: To detect exactly density of smoke in a discharged gas with a small power consumption by controlling intermittently quantity of conducting electricity to a heater of a smoke sensor.

CONSTITUTION: As for a smoke sensor, a pair of platinum electrodes 2a, 2b are formed on the surface of an alumina substrate 1, and a heating element 4 is provided on the rear side, and smoke quantity is detected by measuring electric resistance between the electrodes. This smoke sensor is installed to a gas discharging pipe in which a discharge gas of 200°C flows, and measurement time (time when the sensor is operated) t1, time t2 for making electricity conduct to the heating element, and the time t2 for radiation-cooling between the electrodes are set by using a program timer. A voltage applied to the heating element 4 is set so that the maximum temperature between the electrodes in the course of electric conduction becomes ≥550°C, and the electric conduction is executed for a short time in order to reduce the power consumption. According to such a constitution, the smoke deposited between the electrodes is heated and removed easily at every prescribed time, and the density of smoke in the discharged gas can be detected exactly.