A connector 2b for an NOx sensor is connected via a cable C to an end of a sensor body 2a through which a first pump current and a second pump current flow according to the concentration of oxygen and that of NOx, respectively, contained in a gas to be measured. The connector 2b is provided with not only terminals for inputting a signal to and outputting a signal from the sensor body 2a but also a label resistor RL having resistance corresponding to characteristics (relationship between oxygen concentration and first pump current and that between NOx concentration and second pump current) of the sensor body 2a and label signal output terminals T01 and T02 connected to opposite ends of the label resistor RL. On the basis of characteristics of the sensor body 2a, which are identified from the resistance of the label resistor RL which, in turn, is identified via the terminals T01 and T02, the oxygen concentration and the NOx concentration are accurately obtained from detected values of the first and second pump currents. This enables an NOx sensor to always perform accurate measurement irrespective of variations in characteristics among NOx sensors.