PURPOSE: To execute control having a little hunting and to improve the quality by detecting changing rate of level for actually measured molten surface, inputting this integral value to a nonlinear hysteresis characteristic circuit and correcting control input by correcting power when the changing value is greater than the prescribed value.

CONSTITUTION: A level signal PV for the actually measured molten surface from the detector is inputted to the difference detecting circuit 44 as changing moment by moment and the changing rate for actually measured molten surface level at every unit times in the level signal is outputted. Then, it is integrated by an integrator 45 and the integrated value is inputted to the nonlinear hysteresis characteristic circuit 47 through a switch mechanism 46 and when the changing rate for the actual measured molten surface level is greater than the prescribed value, the correcting power (y) is outputted, to correct the control power.