Abstract: An oxygen concentrator for supplying enriched oxygen gas to an open breathing system of a human being or the like in synchronization with a breathing cycle of a patient is disclosed, comprising a thermocouple sensor and a control device for synchronizing the timing of opening an electromagnetic valve provided in the gas passageway with the beginning of an inhalation phase of the breathing cycle. The thermocouple sensor, located near the patient's nostrils detects the change in temperature of breathing air flow of the human being or the like and generates a detection signal indicating the instantaneous temperature of the breathing air flow. Thereafter, the control devices detects the beginning of an inhalation phase by comparing the successive two voltage levels of the detection signal. When the breathing cycle enters the inhalation phase, the control device opens the electromagnetic valve thereby permitting the flow of oxygen to the patient.