A power-source voltage diagnostic device of an electric power steering apparatus that improves the performance for the anomalous detection of the motor power-source voltage VR-detecting circuit and is capable of continuing the assist control even if the extraordinary state is detected. When the failure (extraordinary state) detection of the VR-detecting circuit is performed by using the correlation between Vbat1-voltage obtained by the detecting section (Vbat1-detecting circuit) to detect the control voltage from the control power-source circuit and VR-voltage obtained by the detecting circuit (VR-detecting circuit) to detect the motor power-source voltage from the power source circuit, multiplexing is performed by providing VR-detecting circuits (VR-detecting circuit 1, VR-detecting circuit 2) of two systems and mutually comparing them so as to accurately perform the failure diagnosis of the VR-detecting circuits.