A 61-year-old man was admitted to our hospital because of a complaint of blood in stool. He was diagnosed with advanced colon and gastric cancers. Computed tomography (CT) revealed a sigmoid tumor with invasion to the bladder, a metastatic tumor in the lateral segmental branch of the left hepatic lobe, and ascites. He was diagnosed with initially unresectable double cancer. Ileostomy was performed immediately, and he was treated with modified (m) FOLFOX6 regimen (oxaliplatin in combination with infusional 5-fluorouracil/Leucovorin). After 6 courses of the mFOLFOX6 regimen, CT revealed that the primary lesion of the sigmoid colon and liver metastasis had reduced in size, and the ascites had disappeared. Gastroscopy revealed that the gastric cancer had disappeared. Biopsy results were negative. Accordingly, his gastric cancer was diagnosed as treatment effect Grade 3. After 8 courses of mFOLFOX6 therapy, sigmoidectomy, partial resection of the bladder, and partial resection of the liver were performed. Gastric cancer was not resected in accordance with his will. Although 40 months has passed after the radical resection, neither the sigmoid colon cancer nor the gastric cancer recurred.