One lung ventilation for thoracic surgery may reduce arterial oxygen saturation by ventilation-perfusion mismatching and by increasing intrapulmonary shunting. We experienced a 52 year old male patient with a lung abscess, who showed arterial desaturation after one-lung ventilation. The authors tried to increase arterial oxygen saturation by applying positive end-expiratory pressure (PEEP) and continuous positive airway pressure (CPAP) to dependent and nondependent lung, respectively. However, arterial oxygen saturation did not increase. So, we applied partial differential lung ventilation using an open-close valve designed by the authors, and arterial oxygen saturation increased rapidly. The advantages of the open-close valve are its straightforward use and ease of manipulation. We report a case involving the use of this open-close valve in a desaturated patient during one lung ventilation.