To suppress cavitation to a minimum, prevent hunting and wear of a valve rod and gland section, and actuate a small diameter valve and a large diameter valve quickly even when pressure of the discharge room is raised rapidly, in an automatic pressure regulating valve.

The automatic pressure regulating valve comprises a valve body which is divided into an inflow room 4 and a discharge room 5 by a partition 2 having a large diameter hole 3, a cylinder 21 which is divided into a pass room 20 and an accumulation room 7 by a piston 16, a large diameter valve 6 which is used to close the large diameter hole 3, a small diameter valve 10 which is used to close a small diameter hole 8 provided on the large diameter valve 6, and a valve rod 18. The inflow room 4 of the valve body is connected to the pass room 20 of the cylinder 21, and the discharge room 5 of the valve body is connected to the pass room 20 of the cylinder via an appropriate flow regulator, a pilot valve or the like, in order to link the inflow room of the valve body to the accumulation room of the cylinder. Besides, the piston in the cylinder is joined to the small diameter valve in the inflow room with a piston rod 13, in order to link the small diameter valve and the large diameter valve movably together, while permitting an independent movement in a prescribed range of the small diameter valve.