An electric power steering apparatus controlling an operation of an electric motor by a control unit based on a torque signal from a torque sensor and executing a steering assist of a steering shaft via a reduction mechanism. The control unit includes a power supply voltage detector for detecting a power supply voltage signal of an in-vehicle power supply, and an output inhibitor for inhibiting an output in multiple stages based on a power supply voltage detected by the detector. A maximum driving current of the motor is limited when the voltage is decreased by the inhibitor to reach a first reference value (V1), a motor control output gain is decreased when the voltage reaches a second reference value (V2), and the output is turned off when the voltage reaches a third reference value (V3).