PURPOSE: To obtain an automatic constant pressure valve device with a simple operation control by which a secondary pressure can be always maintained to a prescribed value without a blocking trouble and a water leak by combining a main valve device consisting of a main valve body and a main valve drive member with a pilot valve device consisting of plural pilot valve parts together.

CONSTITUTION: When a water is flowed by interposing an automatic constant pressure valve device to a fluid transporting pipe route, the pressure of an exit flow route (c) does not reach a prescribed pressure and an A valve body 24 is fully opened and a B valve body 25 is closed, because of that the coil spring 27 force of a pilot valve device is superior to the secondary pressure of a secondary pressure room (i). A primary pressure flow water from an inlet flow route (a) is flowed in the main valve drive pressure room (d) of a main valve device through a communication route P, an A valve output (e), an A valve seat room (f), a communication route Pe. While, when the secondary pressure applied to the pressure receive plate 23 of the pilot valve device overcomes the coil spring 27 force, the A valve body 24 is closed and the B valve body 25 is fully opened and the inner pressure in the main valve drive pressure room (d) reaches the secondary pressure. The main valve body 5 is closed by the difference between the primary pressure and the secondary pressure and the opening (b) of the main valve is shut tightly.