To provide a rear wheel steering device of a vehicle capable of optimizing a behavior of the vehicle during turning by properly controlling rear wheel steering at all times without applying a sense of incongruity to a driver while preventing a system from being complicated.

On the basis of a front wheel lateral force detected by a front wheel lateral force sensor 9f and a vehicle speed detected by a wheel speed sensor 8, a target rear wheel lateral force is calculated by an equation of the motion of the vehicle including a secondary transfer function obtained by simulating a response delay of a rear wheel lateral force from the front wheel lateral force. The rear wheel steering is performed by drive-controlling a steering actuator 7 for a rear wheel 1r to achieve a target rear wheel lateral force.