PURPOSE: To precisely conduct the calibration of a yaw rate sensor in a one in which the steering angle of rear wheels is corrected on the basis of yaw rate signal by calibrating the yaw rate sensor when vehicle speed is nearly zero and steering wheel steering angle is less than a determined value.

CONSTITUTION: A rear wheel steering device 10 for four-wheel steering system has a rear wheel steering mechanism 18 mechanically connected to a front wheel steering mechanism 14 through a transmission shaft 52 to steer rear wheels 16 so as to have a rear wheel target steering angle according to the front wheel steering angle. The rear wheel steering mechanism 18 is provided with a steering ratio variable mechanism 20 controlled by a control unit 22 according to the operating state detected by a vehicle speed sensor 24 and a steering angle sensor 26, and the steering angle of the rear wheels is corrected according to the output of a yaw rate sensor 22. When the vehicle speed is nearly and the steering wheel steering angle is less than a determined value in the control unit 22, or in the state where no yaw rate is generated, the processing for calibrating the yaw rate sensor 25 is conducted.