A 37-year-old man was referred to our hospital with complaints of dyspnea and general fatigue. Chest radiograph and CT scan revealed thickness of bronchovascular bundles in both lungs. In spite of various examinations, the primary lesion was not identified. He received chemotherapy containing TS-1 and cisplatin. Pulmonary lymphangitic carcinomatosis disappeared and the patient achieved a good partial response. He survived for 14 months after the chemotherapy. We believe the combination of TS-1 and cisplatin is one of the attractive options for patients with cancer of unknown primary site.