To assure safe use of an anesthesia apparatus by starting the second phase of inspiring gas and maintaining inspiring gas with fresh gas flow by controlling the anesthesia apparatus in the second phase when respiratory gas flow from a variable capacity gas container stops before inspiring gas is finished.

In operating an anesthesia apparatus, in a control unit 28 a programmed respiration mode is selected according to parameters such as pressure measured by a pressure gauge 30 which is installed at a position close to a patient 4, and operation of the anesthesia apparatus 2 is controlled by the relevant mode. At that time, in the first phase of respiration such as start of respiration a gas flow generating a preset specific gas pressure from a gas container 6 is fed to the patient 4 together with a fresh gas flow. In the second phase of respiration having a diminished gas flow to the patient 4 the gas flow from the container 6 is stopped, and only a fresh,gas flow is fed. In the third phase of respiration having a full-filled gas container 6 the excessive gas is conveyed to an exhaust gas unit through the second valve 22.