A constant flow valve has a chamber (6) provided therein with a first diaphragm (8) having a valve member (7), a second diaphragm (9) and a third diaphragm (10). The chamber is divided into a first pressure chamber (11), a second valve chamber (12), a first valve chamber (13) and a second pressure chamber (18). The first pressure chamber has means for applying a constant inward force to the second diaphragm, and the second pressure chamber has means for applying a constant inward force to the third diaphragm. The first valve chamber is communicated with an inlet channel, the second valve chamber has a valve seat (27) corresponding to the valve member, and the valve seat divides the chamber into a bottom valve chamber (14) communicating with the first valve chamber through a communication hole formed in the first diaphragm and a top valve chamber (15) communicating with an outlet channel.