Pressure regulating device, such as a first diaphragm stage for underwater use, the device comprising: - a body provided with an inlet connected to a breathable high pressure gas source and an outlet for the breathable gas having pressure lower than the input gas pressure, said body being partitioned into at least one high-pressure gas chamber communicating with said inlet, and an intermediate-pressure gas chamber communicating with said outlet, and the intermediate-pressure gas chamber being communicating with the high-pressure gas chamber through a pressure reducing valve, - said pressure reducing valve comprising a valve seat made on the wall partitioning the high pressure chamber from the intermediate pressure chamber and a cut-off composed of a flared head connected to a stem, the flared head cooperating with said valve seat for closing and opening said valve, - said cut-off being displaceable in the two directions along the longitudinal axis of the stem inside the high pressure chamber such that the flared head travels to be alternatively detached and moved away from said seat for opening the valve and travels to move closer and in abutment against said valve seat for closing the valve; - an elastically deformable diaphragm and that can be elastically prevaricated, the diaphragm being exposed to the pressure outside the device and sealingly separates the intermediate pressure chamber from the external environment, the diaphragm controlling a mechanical actuator for pushing the cut-off along the opening direction in addition to the pressure in the intermediate pressure chamber 302, characterized in that - the stem is slidingly and sealingly supported in a guide opening of a wall of the high pressure chamber, opposite to the wall provided with said valve seat, thanks to a sealing gasket, such as an o-ring or the like, said opening not communicating with other parts of the device such that the pressure in said opening is substantially null or however such to not affect the thrust exerted by the cut-off against the valve seat, and wherein the cut-off has a blind hole axially crossing the flared head and the stem until a given depth such as to create a housing seat of a thrust shaft constituting said mechanical actuator. - a body provided with an inlet connected to a breathable high pressure gas source and an outlet for the breathable gas having pressure lower than the input gas pressure, said body being partitioned into at least one high-pressure gas chamber communicating with said inlet, and an intermediate-pressure gas chamber communicating with said outlet, and the intermediate-pressure gas chamber being communicating with the high-pressure gas chamber through a pressure reducing valve, - said pressure reducing valve comprising a valve seat made on the wall partitioning the high pressure chamber from the intermediate pressure chamber and a cut-off composed of a flared head connected to a stem, the flared head cooperating with said valve seat for closing and opening said valve, - said cut-off being displaceable in the two directions along the longitudinal axis of the stem inside the high pressure chamber such that the flared head travels to be alternatively detached and moved away from said seat for opening the valve and travels to move closer and in abutment against said valve seat for closing the valve; - an elastically deformable diaphragm and that can be elastically prevaricated, the diaphragm being exposed to the pressure outside the device and sealingly separates the intermediate pressure chamber from the external environment, the diaphragm controlling a mechanical actuator for pushing the cut-off along the opening direction in addition to the pressure in the intermediate pressure chamber 302, characterized in that - the stem is slidingly and sealingly supported in a guide opening of a wall of the high pressure chamber, opposite to the wall provided with said valve seat, thanks to a sealing gasket, such as an o-ring or the like, said opening not communicating with other parts of the device such that the pressure in said opening is substantially null or however such to not affect the thrust exerted by the cut-off against the valve seat, and wherein the cut-off has a blind hole axially crossing the flared head and the stem until a given depth such as to create a housing seat of a thrust shaft constituting said mechanical actuator.