To detect damage in wiring or the like to enhance reliability, by comparing output voltages each other when conducting mutual switching between a power source terminal and an output terminal in a bridge circuit.

The power source-output terminals 11, 12, 13, 14 are provided in points A, B, C, D in the bridge circuit 10, and a selector switch 17 switched between the first and second switching positions is connected to the terminals 11-14. A voltage between the points C, D is amplified to a voltage signal V1 in the first switching position by an amplifying circuit 24, and a voltage between the points A, B is amplified to a voltage signal V2 in the second switching position. A failure diagnostic circuit 28 determines diagnosis as a trouble when a difference between the voltage signals V1, V2 exceeds a prescribed allowance range ΔV. A circuit or the like for diagnosing the trouble is not required thereby to be arranged in a sensor element 2, a size therefor is reduced, and the damage in a wiring pattern 9, the wiring 22 or the like is surely detected thereby.