To prevent hunting when a main control unit error occurs by prohibiting motor drive with a latch device when a drive prohibition judgement device judges that the motor drive status is in the prohibited range.

When an error occurs at an main control unit 20, if the drive status of a motor 6 goes into the prohibited range, for example because the steering wheel is turned abruptly in the opposite direction, it is judged in the motor drive prohibition judgement portion 31 that the drive status of the motor 6 is in the prohibited range. A flip-flop 32b maintains the H level. Drive of the motor 6 is prohibited by setting the output IL from the sub control unit 30 to the L level. Accordingly, until the main control unit 20 stops output of drive signals to the motor 6, an interlock can be maintained by the sub control unit 30, and this makes it possible to prevent hunting when an error occurs at the main control unit 20.