1. An electric power steering system configured to assist a steering operation, the electric power steering system including a motor as a drive source, the electric power steering system comprising: a steering force assist device configured to apply an assist force; and a control unit configured to control an operation of the steering force assist device by adjusting a drive current that is supplied to the motor, wherein: the control unit calculates a target assist force generated by the steering force assist device based on a steering torque, and executes a feedback control to adjust an actual current value to a current command value that corresponds to the target assist force, the feedback control being executed by multiplying a deviation of the actual current value from the current command value by a feedback gain, and the control unit changes the feedback gain based on separately calculating: (1) a decrease gain based on an assist gradient that is a ratio of a change in the target assist force to a change in the steering torque, and (2) a decrease gain based on a steering speed, and then multiplying the decrease gain based on an assist gradient and the decrease gain based on a steering speed.