An electric power steering system includes an assisting electric motor for assisting steering force according to a vehicle speed and a steering torque, a power supply source and an electric power steering controlling portion for supplying power to the assisting electric motor from the power supply source for controlling the assisting electric motor. The electric power steering controlling portion completely stops the controlling of the assisting electric motor being controlled and stops operating when the power supply source is in a first abnormal state and temporarily stops the controlling of the assisting electric motor being controlled until the power supply source returns to a normal state from a second abnormal state when the power supply source is in the second abnormal state.