To provide an electric power steering device capable of diagnosing a short failure of a power supply relay in general control for assisting the steering of a steering wheel.

The power supply relay provided in a line for supplying power from a power source to a motor via an inverter circuit interrupts current-carrying from the power source to the inverter circuit in the case of an abnormality. A microcomputer diagnoses, in general control for assisting the steering of the steering wheel, a short failure of the power source relay when the steering wheel is not steered (S4: NO). The EPS can diagnose the short failure of the power supply relay without affecting the assist of the steering of the steering wheel by interrupting the current carrying by the power supply relay (S5) when the steering wheel is not steered.