The patient was an 80-year-old woman with combined pulmonary fibrosis and emphysema. She was diagnosed with pulmonary pleomorphic carcinoma in the right upper lobe, which relapsed 18 months after the operation. Computed tomography showed a mass in contact with the posterior wall of the lower part of the stomach. The patient was treated with two cycles of pembrolizumab, but the disease progressed. She was treated with S-1 as second-line therapy, resulting in tumor-shrinking after two cycles. Progression was not observed over the next twelve months. We report a rare case involving S-1 after immune checkpoint inhibitor treatment.