A 59-year-old female was admitted for spontaneous pneumothorax. After evacuation, the chest X-ray film showed a round mass lesion at the left hilum. Following curettage and bronchoalveolar lavage of the left B3c, the mass became smaller, suggesting the diagnosis of round atelectasis. This is the first report of round atelectasis complicated by spontaneous pneumothorax. The mechanism of this case is suggested to resemble that of round atelectasis associated with pleural effusion.