PURPOSE: To easily and accurately adjust molten metal surface by controlling the molten metal surface in a discharge chamber with air quantity by regulating a solenoid valve on an air supplying piping.

CONSTITUTION: A lower limit detecting sensor 20 detects that the molten metal surface is at lower position than the prescribed position P, signal is transmitted to driving circuit 23 and increasing signal is intermittently transmitted to a control valve 14 from the driving circuit 23. A servo motor 15 in the control valve 14 is intermittently reversely rotated, the valve 14 is driven to open degree increasing direction and the air sent from an air source 3 to a pressurized chamber 5 is increased step by step. By this method, the molten metal surface is slowly ascended and when the molten metal surface reaches the prescribed position P, the lower limit detecting sensor 20 detects and stopping signal is transmitted to the control valve 14 through the driving circuit 23. On the other hand, when the molten metal surface does not reach the prescribed position P, intermittent driving of the control valve 14 is continued.