A respiratory circuit for an artificial inspiration ventilator is characterized in that it controls a volume of an O2/air constant flow as being substantially reversely proportional to a positive and negative airway pressures by patient's own respiration. The respiratory circuit comprises a pressure sensor for detecting an inner pressure of the respiratory circuit, a comparator for comparing the inner pressure with reference values prescribed by reference voltages, and a switching circuit responsive to the results of the comparator for switching a valve passing the O2/air constant flow.