An electric power steering apparatus includes a charge-discharge circuit including a circuit charging an auxiliary power supply by turning on and off an MOS-FET for voltage boosting and that selectively configures a first output mode in which an electric power is supplied to a motor by a first electrical conduction path to which a voltage of a battery is applied or a second output mode in which the electric power is supplied to the motor by a second electrical conduction path to which a voltage of a source of the series-connected battery and auxiliary power supply is applied. When a failure of the battery is detected, the electric power steering apparatus connects the first conduction path to an electrical conduction path to ground by continuously holding the MOS-FET in an on-state and drives the charge-discharge circuit to select the second output mode, thereby configuring a circuit to supply the motor with the electric power only from the auxiliary power supply.