To properly perform an abnormality monitoring of a rear wheel steering control device without affecting a rear wheel steering control and without lowering control performance of rear wheel steering.

A maximum steering angle of the rear wheel steering under a normal control is set. At least either one of whether or not a control target value of the rear wheel steering exceeds the maximum steering angle or whether or not a measured value of the rear wheel steering angle exceeds the maximum steering angle is monitored. When the control target value or the steering angle exceeds the maximum steering angle, a decision that a control system is abnormal is outputted.