The present invention provides an apparatus which stably provides highly concentrated oxygen to a patient, regardless of an environmental temperature and/or a highly concentrated oxygen gas flow rate. The present invention is an oxygen concentrator characterized in that an oxygen concentration sensor detecting a concentration of the oxygen gas produced by the oxygen concentrator, a temperature sensor measuring an environmental temperature and/or a flow rate sensor are provided, a control increasing/decreasing a supply airflow volume of the compressed air supply device based on a detected oxygen concentration detected value to maintain an oxygen concentration at a predetermined concentration is performed, and a control device is switched based on a value measured by the temperature sensor and/or flow rate sensor of the apparatus.