To keep vacuum degree constant by feeding back the actual flowing value to the preset suction flowing value thereby controlling a flow rate adjusting means, approaching molten metal to a mold housing chamber and detecting pressure variation caused at the time of bringing a sprue into contact with the molten metal surface.

When the sprue 19 reaches the molten metal surface, the atmosphere flowing in the mold housing chamber 2 through the sprue 19 and the mold 1 is shut off and the vacuum degree in the mold housing chamber 2 is increased by the flowing quantity. At the time of detecting the pressure variation by a pressure sensor 9, a control part 10 judges that the sprue 19 has already reach the molten metal surface, the the pressure is dropped by prescribed quantity, e.g. by the quantity corresponding to the suction to complete the molten metal surface detecting mode. Thereafter, the actual suction mode is started. In the actual suction mode, the vacuum degree is controlled by executing a PID control based on the actual detected pressure by a processor sensor 9 and outputting an operational signal to both of an analog flow rate control valve 5 and a digital valve 6.