Abstract: Respiratory apparatus for a free underwater diver comprising a sealed and deformable respiratory bag in pressure equilibrium with the exterior and into which the diver inspires and expirates. The bag communicates on the one hand with an absorbant means for CO.sub.2 and on the other hand, with bottles of oxygen and a compressed neutral gas, the communication with the bottle of neutral gas being effected through a valve secured to a plate on which the wall of the bag acts when the volume thereof is reduced. At least one sensor of the partial pressure of oxygen is disposed in the bag and delivers a voltage signal proportional to the pressure, and an electrovalve is placed in a circuit connecting the oxygen bottle to the bag.