A 61-year-old woman who had never smoked was given a diagnosis of adenocarcinoma of the lung with multiple pulmonary metastases. Systemic chemotherapy consisting of carboplatin and paclitaxel was not effective, thereafter daily oral administration of gefitinib was initiated. Six days later, bilateral pneumothorax was found. The extent of the pneumothorax was slight and she recovered without drainage within about one month although treatment of gefitinib was restarted. Gefitinib was effective for lung cancer in this case. Bilateral pneumothorax is a rare complication of chemotherapy for lung cancer. Only 3 such cases under treatment with gefitinib were reported in Japan.