PURPOSE: To detect an aimed gas (CH4) with high accuracy, by adding two series resistance and compensating resistance to a bridge circuit connected so that the outputs of both sensors may become differential by two kinds of gas sensors and two resistors.

CONSTITUTION: Resistors π5, π6 are connected in parallel with the first gas sensor π1 and a resistor π2 which are connected in series and a compensating resistor π0 is inserted between a connecting point (c) of the gas sensor π1 and the resistor π2 and a connecting point (b) of the resistors π5, π6. An output V is generated between a connecting point (a) of the second gas sensor π3 with a resistor π4 and the connecting point (b). Resistance values of the resistors π5, π6 are selected so as to satisfy the following equation π5/π6=π1/π2 (=π3/π4) in the air. When either CH4 or H2 is mixed in the air, electric potential of the connecting points c and b, is displaced but the variation rate of the connecting point (b) is lowered by the resistor π0. Then, the sensor π1 desired to cancell for one gas (H2) is made accorded with the sensor π3 by selecting suitably the value of the resistor π0, and the aimed gas (CH4) is detected with high accuracy.