A 20 year-old man was hospitalized with an abdominal mass and abdominal distension. Investigations resulted in a diagnosis of ileus caused by advanced colon cancer with peritoneal dissemination to the pouch of Douglas. Palliative surgery was performed to relieve bowel obstruction and debulk the tumor. Histopathological examination showed that the tumor was a mucinous adenocarcinoma invading the serosa without lymph node metastasis. Ascites collected during the operation was diagnosed as class V. Administration of PSK (3.0 g/day) and UFT (600 mg/day) as adjuvant immunochemotherapy was started postoperatively to achieve tumor dormancy. He has been followed as an outpatient for 2.5 years with no ascites or abdominal symptoms.