PURPOSE: To obtain the smooth motion of a rod by providing a filter at the rod through hole of a case body in the device in the caption wherein a diaphragm is displaced due to the differential pressure between two chambers to control valve opening via the rod.

CONSTITUTION: The subject device is constructed by separating into an atmospheric chamber B and a pressure chamber A by a diaphragm 5, in case positive pressure is introduced to the pressure chamber from an introducing pipe 12 connected to the intake pipe, etc., of an internal combustion engine, which is displaced to the atmospheric chamber B side with resisting to the spring 11. Accordingly, a control valve is operated via a rod 8 to adjust air pressure supplied to an internal combustion engine. A filter is provided at an atmosphere passage hole 4a where the rod 8 goes through a case body 4 is arranged at the side of the atmospheric chamber B of an atmosphere passage hole 4a of the case body 4 where the rod 8 goes through. In this case, a circular mesh spring 15 fixed to the rod 8 is provided in a guide 16 formed cylindrically for absorbing the swing motion of the rod effectively.