The oxygen therapy system receives data for an elapsed time such as a flow rate value of the oxygen gas, and a blood oxygen level or a level of carbon dioxide partial pressure in arterial blood (PaCO2) of a user, from the oxygen supply device which controls a flow rate of the oxygen gas for inhalation based on the blood oxygen level or the carbon dioxide partial pressure in arterial blood (PaCO2) of the user so that the blood oxygen level or level of carbon dioxide partial pressure in arterial blood (PaCO2) is in a prescribed range. The system calculates a proportion of a duration for each oxygen gas flow rate during which the blood oxygen level or carbon dioxide partial pressure in arterial blood (PaCO2) is in the prescribed range from the acquired data in which the oxygen gas flow rate fluctuates with the blood oxygen level or carbon dioxide partial pressure in arterial blood (PaCO2), and displays the calculated data as a histogram or pie graph on a terminal which a healthcare worker such as a physician operates.
