Abstract: The device comprises a respiratory gas generator including means for establishing, during an inspiratory stage and in a supply pipe leading to an inspiratory orifice associated with an expiratory valve, a pressure of a respiratory gas which is substantially constant irrespective of the flow of inspiratory gas. The means for establishing a substantially constant pressure comprise an ejector of the convergent/divergent type which opens onto said supply pipe. In contrast to ventilators having multiple modes of ventilation which are usually intended for patients subjected to intubation or tracheomatization, the device is intended for patients who are connected to the device through a simple mask covering the nose and mouth, a nose mask or a mouthpiece.