A gas concentration detection apparatus having a function of detecting the sensor element activation state

A gas concentration detection apparatus comprising: a sensor element comprising a solid electrolyte layer responsive to an applied DC voltage for performing a concentration indicating current varying in level according to a concentration of a specific gas component in a test object gas, an AC power source and a DC power source for respectively applying an AC voltage; the DC voltage to the sensor element; a current sense resistor connected in series with the sensor element in an AC path through which an AC component flows in response to the applied AC voltage, and a detection signal output circuit for obtaining a gas concentration detection signal indicative of the concentration of the gas component based on the concentration indicative current; and obtaining an impedance detection signal indicative of an impedance value of the sensor element based on the AC component; the detection signal output circuit comprises: a first detection signal output circuit for obtaining the gas concentration detection signal coupled to the current measuring resistor for extracting a first voltage signal, wherein the first voltage signal varies in level according to the concentration indicating current level, and a second detection signal output circuit for obtaining the impedance detection signal; which is coupled to a connection point between the current sense resistor and the sensor element for extracting a second voltage signal, the second voltage signal being an AC signal whose amplitude varies with respect to a circuit ground potential according to an amplitude of the AC component.