When a collision avoidance operation determiner determines a collision avoidance operation by a driver, a target assist electrical current calculator calculates a target assist electrical current based on a deviation between a standard yaw rate corrected in accordance with avoidance momentum calculated by an avoidance momentum calculator and an actual yaw rate; and the target assist electrical current is supplied to a steering actuator to assist the collision avoidance operation by the driver. At this time, when an under-steer determiner determines an under-steer state, an assist electrical current is decreased by a reaction force electrical current calculated in a reaction force electrical current calculator. Therefore, a steering angle is prevented from becoming too large due to excessive assist, thereby facilitating a return operation after avoiding an obstacle.