PURPOSE: To stabilize the switching of a valve at the time of load fluctuation transient by inserting orifices into both pipe paths of a control pipe to the cylinder device upper chamber of an intake throttle valve from an oil separator and the control air relief pipe of the cylinder device upper chamber.

CONSTITUTION: An orifice 19 is inserted into a control pipe 14 to the cylinder device upper chamber 4 of an intake throttle valve 1 from an oil separator 8. An orifice 18 is inserted into the control air relief pipe 15 of the cylinder device upper chamber 4. The pressure of the cylinder device upper chamber 4 does not rise quickly, the vertical shift quantity of a valve 3 is not changed, and no vibration of noise is generated.