The degrees of freedom in the design of a layout structure of a power steering system for a vehicle are increased so that the auxiliary parts can be arranged around the power assisting actuator without the need for a special increase in the strength and in the stiffness of a body frame for installing the power steering system. The front wheels are steered by a power steering system supported by the body frame wherein the power steering system is provided with an actuator for assisting a steering force. One end of the actuator is installed to a first cross pipe laid over the left and right portions of the body frame. The arrangement is such so that a longitudinal direction of the actuator runs substantially in parallel with a center line of a chassis extending in a back-and-forth direction of the vehicle body.