PURPOSE: To stabilize the level of molten metal by performing estimation and correction by using a model generated by representing flow rate disturbance due to nonstationary bulging, etc., as the sum of plural frequencies.

CONSTITUTION: The flow rate disturbance d(t) of the molten metal is assumed as the sum of sine waves of plural frequencies and this is assumed as the control model; and a control command u(t) and a molten metal surface level measured value by a molten metal level sensor are inputted to a filter 21 to obtain functions Z1(t) and Z2(t), which are supplied to a filter 22 and a disturbance estimation arithmetic part 23. The filter 22 finds and supplies a function Φ(t) to the disturbance estimation arithmetic part 23, which finds a disturbance estimated value d(t) hat; and this is multiplied by a correction gain Ko and the product is supplied to an adder 13.