We report the case of a 67-year-old man with a diagnosis of stage IV stomach cancer in May 2010 who was treated with outpatient chemotherapy using TS-1, paclitaxel and lentinan. Dyspnea and coughing developed after drug administration in November and the patient was hospitalized on day 5 after the appearance of symptoms due to hypoxemia and the presence of ground-glass opacities in the right middle and lower lung fields. On the same day, bronchoscopy was performed for differentiation from infection and lymphangitic carcinomatosis. A transbronchial lung biopsy suggested drug-induced pulmonary toxicity, and a drug lymphocyte stimulation test was highly positive for TS-1. Discontinuation of TS-1 alone improved his respiratory status and imaging findings. TS-1 is available only in Japan, and because it is administered orally and its toxicity is minimal, its use has been expanded to treat a variety of malignancies. Drug-induced pulmonary toxicity due to TS-1 occurs in only 0.03% of all cases, and there are few reports regarding the histopathological findings of TS-1-related pulmonary toxicity. Although it can be difficult to diagnose drug-induced pulmonary toxicity because it demonstrates a variety of imaging findings, the present case suggests that it is important to proactively perform transbronchial lung biopsy at the early stage of diagnosis and promptly determine a course of treatment.