Abstract: Intermediate a first pressure reducing means (12, 15) reducing the pressure f a pressure source (10) to nearly the external pressure P.sub.A of the diving depth, and the respiratory mask (18) of the diver, there is a flexible first buffer container (16) whose internal pressure is slightly above the external pressure to ensure that a buffer volume of respiration gas is continuously available directly to the diver. A second buffer container (21) is mounted intermediate the respiratory mask (18) and a second pressure reducing means (26, 23) for waste air. The pressure maintained in the second buffer container is slightly below the external pressure P.sub.A so that a specific exhalation volume is permanently available to the diver. To avoid a direct blowing through the respiratory mask, a relief pressure valve (20) only opens responsive to the lung activity of the diver.