A power steering system includes a torsion bar, which is twistable around its axis for an amount corresponding to a steering torque applied to a steering shaft, a torque sensor, an electric motor, which is driven through a drive circuit to provide the steering shaft with steering assisting force, and motor control means for obtaining a steering torque from a digital signal transmitted from sensor and outputting a drive Duty signal to the drive circuit so that a current suitable for the steering torque is supplied to the electric motor. The torque sensor includes a permanent magnet, a magnetic body, and a Hall IC having a galvanomagnetic element, an AD converter circuit, and a serial communication circuit for digital communications with the motor control means through a communication line and a power supply ground line.