PURPOSE: To improve the precision of stock level control in a bed tank and to stabilize controllability.

CONSTITUTION: The speed of a start conveyor 8 and an end conveyor 10 in the group of conveyors 8, 9 and 10 supplying a powder and a solid body into the bed tank 4 can be varied, and a weighing machine 190 is provided for the start conveyor 8. A microcomputer decides the speed V1 of the start conveyor 8 from the prediction value of tank cutout quantity. Weight per unit length is sequentially tracked from the signal QIN and speed of the weighing machine 19 and the speed V3 of the end conveyor 10 is decided from the weight of a falling point to the bed tank 4 and the operation output of level control. Thus, highly precise and stable level control can be obtained even if the conveyor 3 group is long and level fluctuation is violent.