In PCPS over a long period of time, the water stored in the oxygenator hollow fiber decreases the gas exchange effective membrane area of an oxygenator and causes a gas exchange ability drop (wet lung state). In such a case, we increase to supply with gas of the oxygenator temporarily (gas flash), and improve gas exchange ability. We experimented reduction of wet lung state creating the circuit for an experiment about three kinds oxygenator, silicon coated MERA EXCELUNG PRIME, made with homogeneous membrane structure TERUMO CAPIOX-LX, and NIPRO BIOCUBE. As a result, the drop of the gas exchange ability in the wet lung state was severe in order of BIOCUBE, EXCELUNG, CAPIOX. It was revealed that the effective membrane area could be improved by gas flow quantity more than 25L/min in EXCELUNG, 10L/min in CAPIOX, 30L/min in BIOCUBE. Moreover, improvement of the effective membrane area in all oxygenator was obtained by making gas flow 5 seconds. When using the oxygenator continuously in PCPS for a long time, the results suggested that the gas exchange ability could be improved by effective gas flash.