PURPOSE:To dispense with a return spring and prevent oversteering even in the case of abnormality of a power source system. CONSTITUTION:The device is equipped with motor drivers 5, 6 to rotate a motor to control a rear wheel steering mechanism; a backup micro processor 3 and a first micro processor 1 that can control the motor drivers 5, 6; first and second relays 7, 8 to conduct connection between the first micro processor 1 or the backup micro processor 3 and the motor drivers 5, 6; a first constant voltage regulator 55 for the first micro processor 1; and a second constant voltage regulator 56 for the backup micro processor 3. At the time of first micro processor or first constant voltage regulator abnormality, the first and second relays are changed over to the backup micro processor 3 side, and also third and fourth relays are changed over for a fixed time, and a motor stop signal is transmitted.