An apparatus (1) for non-invasive mechanical ventilation comprises a fan (2) of Bi-level type, or BiPAP, for generating an air flow according to a succession of inspiration steps, or IPAP, and exhalation steps, or EPAP. The flow is conveyed in the aerial ducts of a patient (25) by means of a flexible tube (3) connected to a nasal mask (4). A flexible air reservoir (5) is provided pneumatically connected to the duct (3) and the nasal mask (4) in order to subtract from the inspiration flow a certain air amount before that it reaches the patient (25) at the beginning of an IPAP step. During the exhalation EPAP step, since the air flow pressure decreases automatically, there is the emptying at least partial of the flexible air reservoir (5), whereas the air exhaled by the patient exits through an opening (14) made on the nasal mask (4). This way an air exchange occurs also in the most peripheral patients aerial ducts and the amount of gaseous exchanges is increased, reducing at the same time the duration of the treatment.
