Abstract: In a constant pressure regulator according to the present invention, a diaphragm defining an air chamber with a body is clamped between the body and a cover. An air chamber defining ring provided around the air chamber is clamped between the body and the cover. The upper surface and the lower surface of the air chamber defining ring are formed with a first recess having a seal ring fitted therein for backing the annular protrusion of the diaphragm and a second recess with a seal ring fitted therein for sealing the air chamber, respectively. The diaphragm has a central cylindrical bore portion defining a fluid control potion with the plug formed on the body. A second valve chamber is defined between the diaphragm and the cover. The body is formed with a first valve chamber and an inlet flow passage, and the second valve chamber is formed with an outlet flow passage.