To surely diagnose a failure in switching elements at a stoppage, even if, naturally, a motor is operating.

When the motor 8 is deactivated or operated under PWM control, a CPU 6 controls the switching elements (FET11, FET12, FET13, FET14), based on a switching instruction signal so as to selectively carry a current. A first diagnosis signal T1 at one motor terminal A, detected by a first diagnosis signal detecting circuit 3 and a second diagnosis signal T2 at the other motor terminal B, detected by a second diagnosis signal detecting circuit 4, are inputted. Rise/fall is detected in the inputted first and second diagnosis signals T1, T2, and the failure in the switching elements is diagnosed, based on the detection result.