PURPOSE: To miniaturize a device, simplify the structure therrof, stabilize the flow rate and ensure the operation by a method wherein a shut-off stopper opening and shutting in linkage to the change-over of a venturi for setting the flow rate is provided in the part opened to the air of a critical-flow venturi provided in the downstream of the venturi for setting the flow rate.

CONSTITUTION: The critical-flow venturi 3 one end of which is opened to the air is provided in a passage 2 on the downstream side of the venturi 1 for setting flow rate. An air cylinder 4 is fixed to the venturi 3 through the intermediary of the flange 3d and the bolt 6 thereof. At the tip of the projected piston rod of the air cylinder 4 the shut-off stopper 5 which can be opened and shut is provided in engagement with the sheet surface 3c of the venturi 1. The shut-off stopper 5 is equipped with an electromagnetic valve opened and shut in linkage to the change- over of the venturi 1 and the stopper is opened when the bore of the venturi 1 is small, while it is shut when the bore turns large. Therefore, the flow rate is stabilized, the operation is ensured with no noise, the device is miniaturized, and the structure thereof is also simplified.