When transistors are switching-controlled by a microcomputer, on the basis of the absolute value of a value obtained by subtracting an actual opening angle value from an instruction value generated from the microcomputer for the opening angle of a throttle valve serving as a driven apparatus which is driven by a d.c. motor, a current flowing through each of resistors detected by comparators, and voltages at both ends of a resistor detected via each of the buffers, the microcomputer determines an abnormality in the d.c. motor and the throttle valve and an abnormality in the transistors or the power supply path while distinguishing those from each other.