The ventilation module (8) regulates the inspiratory pressure (P) according to an inspiratory pressure order (AI). A control module (9) compares the breathed volume at each cycle (VTI) with a minimum volume order (VRImini) and varies the pressure order (AI) in the direction tending to maintain the breathed volume (VTI) just over the minimum (VTImini), but keeping the pressure order (AI) within the interval comprised between the two extreme values (AImini, AImaxi). Utilization to combine the advantages of the pressure mode ventilation with those of the volumetric ventilation.