An 89-year-old man was admitted to our hospital for thorough investigation of refractory diabetes mellitus, which revealed primary squamous cell carcinoma of the duodenum. After two courses of chemotherapy, follow-up esophagoduodenogastroscopy and duodenal biopsy showed no evidence of tumor. No findings were suggestive of recurrence of the primary lesion 19 months after starting chemotherapy. This case suggests that chemotherapy including TS-1 may be effective for treating unresectable primary squamous cell carcinoma of the duodenum.