PURPOSE: To stabilize charging operation of the raw material for furnace top by measuring the weight of the raw material in a top bunker during discharge, controlling continuously the opening degree of a gate and determining the average opening degree of the gate so as to be the initial opening degree in the stage of discharging in the next period.

CONSTITUTION: The weight of the charging raw material 2 in a top bunker 1 is continuously measured by a weighing device 6 and is inputted to a calculator 9 and a controller 10. The calculator 9 outputs the signal for the intended residual amt. of the raw material. The controller 10 is inputted with said signal and outputs manipulated variable. The opening degree of a control gate 3 for the raw material is detected by an opening degree detector 5 and is fed back to a sequence controller 7 and the controller 10 by which a selector switch 11 is selected. The controller 10 outputs the difference between the output of the calculator 9 and the signal from the device 6 and corrects the opening degree of the gate 3. The opening degree is continuously corrected and the average opening degree is calculated by the controller 7 which outputs the calculated average opening degree so as to be the initial opening degree of the gate 3 in the stage of starting the next discharge.