Herein, we report a case of 72-year-old man who had L858R EGFR-mutated lung adenocarcinoma. Chest computed tomography revealed a large lung mass that had completely replaced the right upper lobe. Although the mass dramatically shrank after initiating chemotherapy, non-malignant pleural effusion appeared. Because diffuse pleural thickening and shrinking of the thoracic cage gradually became apparent, the patient was diagnosed with trapped lung. Despite the stabilization of his lung cancer, he experienced severe dyspnea and significant weight loss, ultimately leading to a decreased performance status. Chest physicians should recognize that trapped lung can develop as a sequela of dramatic tumor shrinkage in lung cancer.