PURPOSE: To uniformize the product quality by arranging a vacuum tank for holding constant reduced pressure condition, connecting the tank with a suction pipe providing a chamber and a flow rate control valve and controlling pressure reducing speed of the chamber with the flow rate control valve.

CONSTITUTION: A suction connecting hole 5 is formed at the upper part of the chamber 2 and connected with the suction pipe 8. The suction pipe 8 is further connected with another suction pipe 8 through a flexible hose 6 at the upper part thereof and also connected with the vacuum tank 13 by respectively arranging a branching part 9 and the flow rate control valve 12 in the suction pipe 8. A sprue 4 is dipped into molten metal and a solenoid valve 11 is opened, to suck air in the chamber 2 into the tank 13. Then, the flow rate control valve 12 executes the pressure control at the aimed pressure given with an adjusting meter 20 and recorder 21. As variation of the pressure in the chamber 2 is restrained, misrun and development of creasy surface of the product are prevented. Therefore, the product quality is uniformized.