PURPOSE: To accomplish a vacuum suction in a mold cavity in a remarkably effective manner by a simple constitution by omitting the loading of an air tight material which is so disposed as to surround the mold cavity and to immediately apply the present device even to an existing injection mold.

CONSTITUTION: A sealing plate 34 is mounted on the outer surface of a spacer block 20 or a movable mold 12 for sealing a space part 30 provided in the spacer block 20 from the external. A connection port 38 for a vacuum sucking pipe 36 is provided in a part of a movable mounting plate 22 so as to be made directly communicate with the sealed space part 30. The connection port 38 is connected to a sucking port 42 of a vacuum pump 40 through the vacuum sucking pipe 36 via a pressure reducing tank 44. In this manner, the space part 30 is brought into a vacuum state under the action of the pressure reducing tank 44. The accomplished vacuum state is made to act to a mold cavity 14 through ejector pin inserting holes 26 provided in the movable mold 12. In this manner, the mold cavity 14 can be breathed in an effective and positive manner.