The present invention includes a control part for controlling switching elements on diagonal lines in the upper and lower stage of an H bridge circuit respectively by different duty ratios based on a steering torque exerted on a steering system, and an electric power supply source for supplying an electric power. When the voltage of the electric power supply source is lower than a prescribed voltage, the control part causes the switching element EET 1 or FET2 on the upper-stage side to be driven by a pulse width modulation signal PWM1 or PWM2, the switching element FET 4 or FET3 on the lower-stage side to be turned on, and the frequency of the pulse width modulation signal to be lowered to an audible frequency range.