PURPOSE: To improve the yield of product by arranging a buffer tank having variable capacity so as to communicate within a furnace.

CONSTITUTION: A pre-load tank 45 and a pressure proportional control valve 42 are arranged in control circuit 18. This control circuit 18 is arranged so as to communicate within the furnace 12 through piping. By negative pressure control in the control circuit 18, the molten metal melting base alloy 11 is held in the furnace 12. At the time of tapping the molten metal from the furnace, the pressure in the furnace is changed from the negative pressure to positive pressure. Then, the buffer tank 50 having variable capacity is arranged so as to communicate within the furnace 12. By this method, operational ratio of the equipment is improved.