An oxygen concentrator is provided with a controller for recovering an oxygen concentration to a level suitable for treatment in a short period of time by selecting an optimum purge time corresponding to the deterioration state of an adsorbent. The judgment of moisture-absorption deterioration is performed when the detected value of the oxygen concentration sensor is equal to or less than a control value of the oxygen concentration in the oxygen-enriched gas and the detected value of the pressure sensor is equal to or more than an adsorption pressure at which the oxygen concentration increases significantly before and after the control to reduce the purge time, and control of reducing a time for the purge step shorter than a preset time is performed.