PURPOSE: To efficiently, continuously perform melting treatment of incineration ash without using auxiliary fuel by a method wherein phase output terminals of a multi-phase alternating current power source are connected to discharge electrodes of an electrode unit clockwise or anticlockwise in the order of early phase.

CONSTITUTION: Single phase alternating currents A1-A6 are output from single phase transformers S1-S6 of a six-phase alternating current output device and among the six single alternating currents, three single alternating currents which are in a relation of phase difference of 120° to each other energize discharge electrodes T1-T6 alternately. Further, two among the six single phase alternating currents A1-A6 output from the six-phase alternating current output device, which are in an inverse phase relation to each other, energize the opposite ones among the discharge electrodes T1-T6. That is, the phase output terminals of six phase alternating current are connected to the six discharge electrodes T1-T6, whose tips are closely arranged at the positions of the vertexes of a regular hexagon, clockwise or anticlockwise in the order of early phase. Thus, the discharge directions of fifteen arcs between the six electrodes can be aligned.