PURPOSE: To stabilize a molten metal surface level in a mold by calculating mold width changing quantity with molten metal surface control in a continuous casting device and correcting the opening degree of a tundish stopper based on it.

CONSTITUTION: The width change shifting quantity in a mole width changing device 9 is calculated, and varying quantity of the molten metal surface in the mold is derived with arithmetic circuit 10 in an arithmetic unit, and opening degree correcting value is obtd. with opening degree correcting circuit 11 from this output. Then, the molten metal surface value from a level detector is corrected with this opening degree correcting value and the molten metal surface level in the mold is controlled with a stopper position control circuit 6 for controlling a stopper driving device 5. By this method, even in the case of changing the mold width, the stale molten metal surface level control can be executed.