To facilitate maintenance of a furnace body, and to improve the service life by maintaining a heating quantity even if source voltage drops, performing stable operation in a specific processing time, improving a heating balance of respective electrodes in a three-phase AC electric furnace, and eliminating local overheat of a furnace wall.

The three-phase AC electric furnace impresses a three-phase alternating current on the three electrodes A, B, and C suspended in the furnace 2, and independently controls the respective electrodes in constant impedance by automatically lifting the electrodes by electrode lifting devices 3a, 3b, and 3c so that the ratio of voltage to an electric current becomes constant on the respective electrodes by respectively detecting the voltage and the electric current impressed on the respective electrodes, and arithmetically operates a required correction value from a fluctuation quantity by detecting a fluctuation in the voltage or the electric current impressed on the respective electrodes, and reduces a fluctuation input electric power by adding the correction value as an operation quantity of the electrode lifting devices.