To provide an automatic electric power control device of an arc furnace in which a desired furnace injection electric power can be maintained always even if a power supply voltage fluctuates greatly.

This device is provided with an instrument transformer 52 to detect an electrode voltage VFB of the arc furnace 3, a voltage control circuit to change the electrode voltage VFB by selecting alternatively a plurality of taps of the transformer 2 for a furnace, and a judgment part to actuate the voltage control circuit so as to return the electrode voltage VFB back within a prescribed range in the case the electrode voltage VFB exceeds downward or upward a prescribed range under the selected tap.