Apparatus for delivering therapeutic gas, comprising: - an internal gas passage in fluidic communication with a deformable reservoir in order to feed the deformable reservoir with therapeutic gas and to recover therapeutic gas coming from said deformable reservoir, - a valve device arranged on the internal gas passage, upstream of the deformable reservoir, in order to control the flow rate of therapeutic gas feeding the deformable reservoir, - a control unit with microprocessor controlling the valve device in order to set or adjust the flow rate of gas passing through said valve device, - flow rate determination means designed to perform one or more measurements of pressure or flow rate in the internal gas passage and to transmit said measurement(s) to the control unit, - a pressure sensor configured to perform one or more gas pressure measurements (P 55) on the therapeutic gas feeding the deformable reservoir and to supply said gas pressure measurement (s) (P 55) to the control unit, and - a breathing mask in fluidic communication with the internal gas passage and fed with therapeutic gas coming from the deformable reservoir, wherein - the pressure sensor is arranged on the internal gas passage between the valve device and the deformable reservoir, and - the control unit is configured to: a) determine the flow rate of gas (Q 60) circulating in the internal gas passage on the basis of the one or more pressure or flow rate measurements supplied by the flow rate determination means, b) determine the gas pressure (P 55) on the basis of the one or more gas pressure measurements (P 55) coming from the pressure sensor, c) determine the losses of head (ΔP) downstream of the pressure sensor on the basis of the gas flow rate (Q 60), d) determine the pressure (P M) at the breathing mask on the basis of the gas pressure (P 55) and of the losses of head (ΔP), e) and control the valve device to adjust the gas flow rate according to said pressure (P M) determined at the breathing mask, in such a way as to adjust the pressure at the breathing mask to a value equal to atmospheric pressure. - an internal gas passage in fluidic communication with a deformable reservoir in order to feed the deformable reservoir with therapeutic gas and to recover therapeutic gas coming from said deformable reservoir, - a valve device arranged on the internal gas passage, upstream of the deformable reservoir, in order to control the flow rate of therapeutic gas feeding the deformable reservoir, - a control unit with microprocessor controlling the valve device in order to set or adjust the flow rate of gas passing through said valve device, - flow rate determination means designed to perform one or more measurements of pressure or flow rate in the internal gas passage and to transmit said measurement(s) to the control unit, - a pressure sensor configured to perform one or more gas pressure measurements (P 55) on the therapeutic gas feeding the deformable reservoir and to supply said gas pressure measurement (s) (P 55) to the control unit, and - a breathing mask in fluidic communication with the internal gas passage and fed with therapeutic gas coming from the deformable reservoir, wherein - the pressure sensor is arranged on the internal gas passage between the valve device and the deformable reservoir, and - the control unit is configured to: a) determine the flow rate of gas (Q 60) circulating in the internal gas passage on the basis of the one or more pressure or flow rate measurements supplied by the flow rate determination means, b) determine the gas pressure (P 55) on the basis of the one or more gas pressure measurements (P 55) coming from the pressure sensor, c) determine the losses of head (ΔP) downstream of the pressure sensor on the basis of the gas flow rate (Q 60), d) determine the pressure (P M) at the breathing mask on the basis of the gas pressure (P 55) and of the losses of head (ΔP), e) and control the valve device to adjust the gas flow rate according to said pressure (P M) determined at the breathing mask, in such a way as to adjust the pressure at the breathing mask to a value equal to atmospheric pressure. a) determine the flow rate of gas (Q 60) circulating in the internal gas passage on the basis of the one or more pressure or flow rate measurements supplied by the flow rate determination means, b) determine the gas pressure (P 55) on the basis of the one or more gas pressure measurements (P 55) coming from the pressure sensor, c) determine the losses of head (ΔP) downstream of the pressure sensor on the basis of the gas flow rate (Q 60), d) determine the pressure (P M) at the breathing mask on the basis of the gas pressure (P 55) and of the losses of head (ΔP), e) and control the valve device to adjust the gas flow rate according to said pressure (P M) determined at the breathing mask, in such a way as to adjust the pressure at the breathing mask to a value equal to atmospheric pressure.