A semiconductor gas sensor includes a CMOS inverter which is configured by an n-channel field effect transistor having a catalytic gate and a p-channel field effect transistor having the catalytic gate. An input setting gate potential Vin(D) of the CMOS inverter is set to satisfy “Vin(D)=Vtc−ΔVgth” by using the sensor response threshold intensity ΔVgth determined by a concentration of a gas to be detected and a threshold input potential Vtc of the CMOS inverter. Therefore, only by setting the input setting gate potential Vin(D), a warning or an alarm can be issued for the concentration of the gas to be detected. In addition, a temperature compensation of the threshold voltage caused by a MOS structure is reduced regardless of the detection gas by setting a characteristic coefficient βR, a threshold voltage Vtn of the n-channel field effect transistor, and the threshold voltage Vtp of the p-channel field effect transistor so as to satisfy relations “βR=1” and “Vtp=−Vtn”.