An object is to perform accurate diagnosis of a failure in which an input signal from an amplification circuit assumes an anomalous value in an intermediate range between GND and Vcc. When at least one of a first NO x concentration Ip2W based on the output OP1 of a first differential amplification circuit 210 and a second NO x concentration Ip2N based on the output OP2 of a second differential amplification circuit 220 falls within an anomaly diagnosis range ZN, the difference D between the first NO x concentration Ip2W and the second NO x concentration Ip2N is obtained, and the difference D is stored in a difference accumulation buffer BF such that 16 values of the difference D continuously sampled up to the present are stored. The integrated value (difference integral value) Dint of the difference D is obtained. When the difference integrated value Dint becomes equal to or greater than a predetermined value Di0, a diagnosis result of "anomalous" is provided.