PURPOSE: To prevent the generation of vibrational and impulsive phenomena by providing a gentle stream chamber for receiving the flow in a valve box and controlling the pressure in the gentle stream chamber by means of an automatic throttle adjusting passage.

CONSTITUTION: A slide valve edge side member 10 pressed toward a sack chamber (g) by means of an equalizing spring is provided on the downstream side of a switching valve 6. An entrance passage (a) is communicated with the sack chamber (g), and a gentle stream chamber (c) is formed on the downstream side of the switching valve 6. The flow passed through the switching valve 6 is flowed into the gentle stream chamber, and the pressure in the gentle stream chamber is controlled by means of an automatic throttle adjusting passage (d). As a result, the generation of vibrational and impulsive phenomena can be prevented.