Abstract: A respiration-synchronized oxygen supplying apparatus includes a sensor detecting respiration of human being, a valve connecting and disconnecting communications between a first gas passage coupled to the human being and a second gas passage coupled to the oxygen supplier, and a controller for controlling the valve based on information from the sensor. A negative pressure generating mechanism is provided having a communication area widened as closer to the human being between the valve and the human being, and a third gas passage is coupled to the negative pressure generating mechanism to normally communicate between the sensor and the human being. During an oxygen supplying state, oxygen does not leak toward the negative pressure sensor, so that the oxygen can be employed without waste.