PURPOSE: To increase the safety of the apparatus by detecting also the abnormality of a semiconductor gas sensor which detects a gas concentration.

CONSTITUTION: The apparatus is provided with the following: a circuit which detects a gas concentration by utilizing a semiconductor gas sensor 1 whose resistance value is changed according to the gas concentration in the circumference and by utilizing the inverting amplification of a single-power-supply operational amplifier 4; and a circuit which detects the disconnection of a heater coil 2 built in the semiconductor gas sensor 1 by using the circuit which detects the gas concentration while the noninverting input voltage and the amplification degree of the single-power-supply operational amplifier 4 is changed over. In addition, the title apparatus is provided with an alarm device 18 which generates an alarm when the gas concentration is at a preset value or higher preset by the circuit which detects the gas concentration and when the disconnection of the heater coil 2 is detected by the circuit which detects the disconnection.