An electric power steering apparatus comprising: a motor configured to apply assist force to a steering system; a torque sensor configured to detect steering torque applied to the steering system; an inverter device including a semiconductor switching element (41a-41f) and configured to drive the motor by driving the semiconductor switching element (41a-41f); a gate driver power source configured to supply power to a gate drive circuit that drives the inverter device; and a controller configured to control the inverter device based on the steering torque, wherein, based on a difference between the voltage (Vg) of the gate driver power source and the power source voltage (Vb) of the inverter device, the controller switches assist control, for applying the assist force to the steering system, to assist stop control, for stopping the assist control wherein: when the indication of the voltage decrease in the gate driver power source is given by a difference between the voltage (Vg) of the gate driver power source and the power source voltage (Vb) of the inverter device; and if the voltage (Vg) of the gate driver power source is equal to or higher than a voltage threshold value (β 0) with respect to the power source voltage (Vb) of the inverter device, the controller exerts assist control, and if the voltage (Vg) of the gate driver power source is not equal to or higher than the voltage threshold value (β0) with respect to the power source voltage (Vb) of the inverter device, the controller exerts assist stop control, wherein the difference (Vg-Vb) is compared with the threshold value (β0), and the assist stop control is executed or a regular assist control is executed depending on the result of comparison. a motor configured to apply assist force to a steering system; a torque sensor configured to detect steering torque applied to the steering system; an inverter device including a semiconductor switching element (41a-41f) and configured to drive the motor by driving the semiconductor switching element (41a-41f); a gate driver power source configured to supply power to a gate drive circuit that drives the inverter device; and a controller configured to control the inverter device based on the steering torque, wherein, based on a difference between the voltage (Vg) of the gate driver power source and the power source voltage (Vb) of the inverter device, the controller switches assist control, for applying the assist force to the steering system, to assist stop control, for stopping the assist control wherein: when the indication of the voltage decrease in the gate driver power source is given by a difference between the voltage (Vg) of the gate driver power source and the power source voltage (Vb) of the inverter device; and if the voltage (Vg) of the gate driver power source is equal to or higher than a voltage threshold value (β 0) with respect to the power source voltage (Vb) of the inverter device, the controller exerts assist control, and if the voltage (Vg) of the gate driver power source is not equal to or higher than the voltage threshold value (β0) with respect to the power source voltage (Vb) of the inverter device, the controller exerts assist stop control, wherein the difference (Vg-Vb) is compared with the threshold value (β0), and the assist stop control is executed or a regular assist control is executed depending on the result of comparison. when the indication of the voltage decrease in the gate driver power source is given by a difference between the voltage (Vg) of the gate driver power source and the power source voltage (Vb) of the inverter device; and if the voltage (Vg) of the gate driver power source is equal to or higher than a voltage threshold value (β 0) with respect to the power source voltage (Vb) of the inverter device, the controller exerts assist control, and if the voltage (Vg) of the gate driver power source is not equal to or higher than the voltage threshold value (β0) with respect to the power source voltage (Vb) of the inverter device, the controller exerts assist stop control, wherein the difference (Vg-Vb) is compared with the threshold value (β0), and the assist stop control is executed or a regular assist control is executed depending on the result of comparison.