A 73-year-old man was referred to our hospital with complaints of constipation and defecation caused by a sigmoid colon tumor. After examination, he was diagnosed as sigmoid colon cancer with lung metastasis, peritoneal dissemination and early gastric cancer. To prevent bowel obstruction, a sigmoid colon resection was performed. On postoperative day 20, S-1 (100 mg/body for 4 weeks followed by 2 drug-free weeks) treatment was started. After 13 courses of treatment, gastrointestinal fiberscope revealed that the gastric cancer was remarkably reduced, and after 16 courses, computed tomography revealed that the lung metastasis was remarkably reduced. Although after 12 courses, elevation of bilirubin, the treatment could be possible to continued on a lowered dose of S-1 from 100 mg to 80 mg/body. Twenty-four months after the operation, good control of cancer has been maintained, and the treatment is continuing. S-1 treatment was proved effective for double cancer of the advanced colorectal cancer and the gastric cancer.