To make holdable the pressure of gas so as to be always constant even when the flow rate of gas to be controlled is changed.

An inlet port 22 and an outlet port 24 are formed at a main body 20, and a spool 40 is held by a spring 28 on sliding faces 29a and 29b. A pilot chamber 30 is formed at the lower part of the spool 40, and the inside pressure of the pilot chamber 30 is held so as to be higher than the pressure of gas in the inlet port 22. A pressure sensor S for detecting the pressure of gas in the inlet port 22, a control valve V for supplying gas for adjustment to the pilot chamber 30 and a control circuit C for driving the control valve V according to the output of the pressure sensor S are formed outside the main body 20. Then, the pressure of gas in the inlet port 22 detected by the pressure sensor S is fed back to the control circuit C, and as a result, the control valve V is driven so that the gas for adjustment can be supplied to the pilot chamber 30.