According to the present invention, an artificial lung in a circulation apparatus can be monitored and be maintained in a safe condition without manual assistance. Therefore, as an extracorporeal circulation mode starts, it is determined first whether or not gas exchange of the artificial lung is carried out within a normal range, based on oxygen concentration which is detected by an oxygen sensor positioned at a downstream place in the artificial lung. If the gas exchange is carried out within the normal range, an estimated value for gas supply volume of a gas blender is maintained. When oxygen concentration exceeding the normal range is detected, the gas blender is controlled so as to revise the gas supply volume downward. In addition, when oxygen concentration falls below the normal range, the gas blender is controlled so as to revise the gas supply volume upward.
