An apparatus for producing oxygen enriched air from the atmospheric air, including an oxygen enriching system employing a fan for inducing a flow of atmospheric air through an oxygen enriching unit and a pump for deriving the oxygen enriched air from the oxygen enriching unit, a sound-proof box for encasing the fan and pump, an air intake passage having not less than five bends and an air exhaust passage also having not less than five bends. The oxygen enriching unit is provided with a unit for adjusting an oxygen concentration to a constant level regardless of any change in the atmospheric temperature.