a first zeolite bed connected to an external air pressurizing means and a depressurizing means for increasing or decreasing an internal pressure thereof;

a second zeolite bed connected to the first zeolite bed in parallel and connected to the external air pressurizing means and the depressurizing means for decreasing or increasing an internal pressure thereof, the first and second zeolite beds being arranged such that the internal pressure of the second zeolite bed is decreased when the internal pressure of the first zeolite bed is increased, and the internal pressure of the second zeolite bed is increased when the internal pressure of the first zeolite bed is decreased;

a carbon molecular sieve bed communicated in fluid with the first and second zeolite beds for receiving and adsorbing oxygen produced in the first and second zeolite beds and discharging the produced oxygen to the outside and discharging selectively some of the produced oxygen to the first zeolite bed or the second zeolite bed;

a first valve section for supplying air from the external air pressurizing means to the first zeolite bed or the second zeolite bed;

a second valve section for supplying vacuum from the air depressurizing means to the first zeolite bed or the second zeolite bed;

a third valve section connected to the first and second zeolite beds and the carbon molecular bed for supplying oxygen generated in the first zeolite bed or the second zeolite bed to the carbon molecular sieve bed, discharging non-adsorbed oxygen in the carbon molecular bed to the first zeolite bed or the second zeolite bed, and equalizing the pressure in the first and second zeolite beds; and

a fourth valve section connected to the carbon molecular sieve bed, an external device and the first and second zeolite beds for discharging selectively oxygen generated in the carbon molecular bed to the external device, the first zeolite bed or the second zeolite bed.