To highly precisely control the flow rate of fluid circulating through a valve body through first and second ports.

For a valve body 16 having a first port 12 connected to a processing chamber 37 and a second port 14 connected to a vacuum pump 39, a first valve seat part 36 where the first valve body 30 is seated on an internal wall face faced to the first port 12 and a second valve seat part 36 where a second valve body 34 is seated in the external direction of the radius of the first valve seat part 32, are formed. Then, a piston 20 formed in a guide body 22 is displaced by pressure fluid so that the second valve body 34 can be isolated from the second valve seat part 36, and that fluid can circulate between the first valve body 30 and the first vale seat part 32. An isolated distance between the first valve body 30 and the first valve seat part 32 is changed under a driving action by a driving part 24 so that the flow rate of the fluid circulating from the first port 12 to the second port 14 can be controlled.