A backflow prevention device with reduced pressure zone to be installed in water systems is provided and comprises a body provided with inlet, outlet and exhaust openings, a check assembly with a stem, an elastic element, and a plate, a mobile diaphragm or membrane adapted to interrupt the fluid connection between the outlet opening and inlet opening and between said inlet opening and exhaust opening, wherein said stem of said check assembly is stabilized with respect to said body, and said plate is slidably disposed along the stem and is cooperatively placed with the mobile diaphragm or membrane in order to interrupt the fluid connection between said inlet opening and said outlet opening.