Abstract: A back pressure control valve which accurately controls pressure of fluid on a primary side but is not affected by pressure variations on a secondary side. The back pressure control valve 10 has a first chamber 12, a second chamber 14, a flow path 16 with a valve seat 17, a valve mechanism 20 with a rod portion 21 with a valve portion 22, a first diaphragm 30 within the first chamber which is provided with a first pressure adjusting mechanism M1, and a second diaphragm 40 within the second chamber which is provided with a second pressure adjusting mechanism M2.

Abstract: A first pressure chamber and a second pressure chamber are provided with means for continuously exerting a constant inward pressure on a first diaphragm part and a third diaphragm part respectively. A valve chamber having a flow control portion wherein a flow rate of the controlled fluid flowing from an inflow chamber to an outflow chamber is controlled by changing a size of the opening defined between a valve part and a valve seat, caused by a displacement of the valve part of a valve mechanism. A differential pressure chamber connects with the outflow chamber and has a bypass opening of a bypass passage connecting to the outflow passage.