PURPOSE: To change the agitating speed for molten metal and the penetration depth of magnetic fluxes in the molten metal to prescribed values according to the level of the molten metal by changing the frequencies of the electric power to be applied to the inductor for agitation of molten metal disposed in the bottom part of a furnace body which holds the molten metal according to the level of the molten metal.

CONSTITUTION: For example, in the case of charging Al scraps into a reverberatory furnace 1 and melting the same, high speed agitation is recommended and when the total accumulated amt. of the molten metal in the furnace increases, low speed agitation is preferred. Thereupon, a level sensor 11 outputs a level signal of molten metal 2, and according to said signal, a setter 12 outputs a signal for regulation of frequencies. The alternating electric power of adequate variable frequencies meeting the amt. of the molten metal is supplied from a control circuit 14 for electric power to an inductor 4. When the level of the molten metal is low, the output frequency to the inductor 4 is increased by the output of the sensor 11, and the progression speed of the shifting magnetic field generated from the inductor 4 is fast and the penetration depth to the molten metals shallow. When the level of the molten metal rises, the frequencies are made low and the opposite takes place. Thus the efficiency of melting is improved, the accelerated oxidation of the molten metal is prevented and the efficiency of the agitation is improved.