PURPOSE: To improve efficiency and to execute precise control by determining the relation between suction time and valve opening degrees from initially set data and the relation between suction pressure and the valve opening degrees of suction speed control valves and controlling the vacuum suction speeds.

CONSTITUTION: The relation between the suction time and suction pressure in the stage of executing the 1st vacuum suction casting is set with a setter 13 and is transferred to a controller 15. The relation between the suction time and suction pressure is set in a storage device of the controller 15 and the relation between the suction time and the valve opening degrees is determined by approximation. A casting mold is dipped in a molten metal in succession thereto and the evacuation of the space in the mold is started. The data on the suction pressure and valve opening degrees is measured at specified time intervals during the evacuation and is stored in the controller 15 which determines the relation between the suction pressure and the valve opening degree, then the relation between the suction time and the valve opening degrees. The relations of the motor rotating speeds N1, N2... at the suction time t1, t2... are calculated by the controller 15 in accordance with the data on plural pieces of the suction time t1, t2... and the respective valve opening degrees K1, K2.... A variable speed motor 12 is then controlled. The suction pressure for the prescribed time is thus controlled.