PURPOSE: To promote a saving of electrical power caused by a reduction in a thermal radiation and at the same time to prevent dust from being diffused or dispersed at a melting surface by a method wherein a feeding electrical power is controlled in response to a level of mattes to be processed and accumulated in a furnace and a sensed temperature at a top part of the furnace, or to their sensed values in such a manner as the temperature at the top part of the furnace may become less than a desired value.

CONSTITUTION: Level meters 8A and 8B will detect a level of a waste material 5 under a variation of torque which is attained by ascent or descent of a weight 8b fixed to a wire 8a with a winding electric motor 8c. Thermometers 10A and 10B will output a sensing signal for causing the weight 8b to be retracted to the top part 1b of the furnace when a temperature more than 800°C is sensed. Output signals from the level meters 8A and 8B, and the thermometers 10A and 10B are fed to a control device 20 and an electric power for main electrodes 4A, 4B and 4C is controlled stepwise through a power supply 30. Therefore, even if the waste material shows a better aeration characteristic, the temperature at the top part of the furnace can positively be restricted to a value less tan the predetermined value. With this arrangement, a thermal radiation at the melting surface can be reduced and a saving of the electric power can be attained and at the same time a distribution of the dust from the melting surface can be reduced.