A 79 year-old man sought treatment in the emergency room complaining of persistent fever, chest pain, and general fatigue. A chest X-ray showed a giant infectious bulla (24 cm in diameter) in the left lung. The patient had no history of abnormalities on X-rays, and his latest medical check-up, conducted in the preceding year, had produced no abnormal findings. Diagnostic procedures, including bronchoscopy, revealed lung cancer (large cell carcinoma) in the left lower bronchus. The tumor obstructed the airway. Although there have been various reports of giant bullae, their etiology remains unknown. We suggest that an obstruction, such as that caused by the tumor in this case, can lead to air trapping, resulting in the formation of a bulla. In the case of a giant bulla that rapidly increases in size, lung cancer should be included in the differential diagnosis.