PURPOSE: To expedite the formation of a molten slag bath and to make the solidified slag after treatment reusable as it is by utilizing microwaves for melting of a slag forming agent.

CONSTITUTION: A slag forming agent S is charged into a die 1 and microwaves are irradiated to this through a waveguide 3 from a microwave oscillator 2, thereby melting the agent S. When a non-consumable electrode 4 is inserted to this and conduction of electricity is started, the molten slag is maintained at a high temp. If a metallic material M is charged therein, the molten metal M settles below the molten slag bath S' on account of a difference in specific gravity and is remelted easily by the resistance heat of the bath S'. Therefore, if the oscillator 2 and the electrode 4 are disposed in adjacent positions and these are so disposed as to be made movable between the die 1 and both, the melting of the agent S and the remelting of the material M are accomplished smoothly.