We have experienced a patient with signet ring cell cancer of the duodenum. The tumor was unresectable and the patient was treated with combined therapy resulting in a decrease of serum level of CEA and 26 month-survival. A 58-year-old man presented with abdominal discomfort and high levels of serum CEA and CA19-9. We could not make the patient recognize his disease but continued to persuade him, and made him admit 11 months after the onset. Exploratory laparotomy indicated H₀P₁3T₃(SE)N₄ according to The General Rules for Gastric Cancer Study in Surgery and Pathology, H₀P₁M_(-)SEN₄₍₊₎ according to The General Rules for Colorectal Cancer Study in Surgery and Pathology. Radiation therapy (50Gy) and 2 times of chemotherapy (CDDP20mg for 5 days) were performed followed by 5 times of chemotherapy (CDDP10mg+5-FU250mg for 5 days) 13 months after the onset and 5 times of chemotherapy (FT 400mg) and oral chemotherapy (5-FUs). The serum level of CEA was dramatically decreased from 574mg/ml at the start of combined therapy and image findings showed no enlargement of the lesion nor development of new lesion. As the serum level of CEA was increased to 20.4mg/ml 19 months after the onset, 2 times of combined therapy with chemotherapy (CDDP+5-FU for 5 days) and thermotherapy were conducted on the day 3. However, the patient died from cancer 26 months after the onset, or 13 months after the treatment