A PSA mechanism has a plurality of adsorption towers having inlet/outlet ends connected to respective valves that are connected to an off-gas discharge passage. The off-gas discharge passage is connected to an off-gas ejector of a residual gas supply. The off-gas ejector has a function to draw an off gas from the PSA mechanism with compressed air that is caused by an air compressor to flow from an off-gas discharging air supply passage into an off-gas passage.