A first diaphragm and a second diaphragm are disposed between a valve element and a body, a diaphragm chamber is formed between the first diaphragm and the second diaphragm, the first diaphragm separates a valve chamber from the diaphragm chamber, and the second diaphragm separates the diaphragm chamber from a back pressure chamber. The diaphragm chamber communicates with an output port, the valve chamber and the back pressure chamber communicate with an input port. A difference between an effective pressure receiving area of the first diaphragm and an effective pressure receiving area of the second diaphragm is equivalent to a passage area in a valve seat.