A power steering system for a vehicle includes a power assist unit composed of a unitized combination of a torque sensor, an electric motor and a power transmission mechanism. The power assist unit is mounted to a cross member provided on the vehicle body side, to enable a reduction in the number of component parts and realization of a compact design. A steering shaft is rotatably supported on only a single support case mounted to a cross member. At least an electric motor of a power assist unit is mounted to the cross member through the support case.