To provide an electric power steering system capable of improving vehicle's kinetic performance.

A step-up goal setting section 107 sets a step-up target value based on a steering torque and the like. A step-up voltage controller 110 executes feedback control and determines a voltage instruction value, based on a voltage difference between the step-up target value and an actual step-up voltage, and outputs a step-up circuit driving duty signal of a duty ratio in accordance with the voltage instruction value. In contrast, a step-up response characteristic correction controller 113 extracts a natural frequency component of vehicle motion from a voltage indication waveform indicating the step-up target value and determines a correction duty signal based on the signal obtained by that extraction. Then, the step-up circuit driving duty signal is corrected with the correction duty signal, and a step-up circuit 111 is controlled according to the corrected signal.