A 76-year-old man visited our hospital with complaints of appetite loss and diarrhea. Abdominal computed tomography (CT)showed a large transverse colon tumor at the splenic flexure and a gastrocolic fistula. Upper gastrointestinal series and gastroscopy demonstrated a type 2 tumor in the transverse colon and a gastrocolic fistula as the scope was inserted into the transverse colon. Colonoscopy showed a type 2 tumor of the rectum. Based on the diagnosis of advanced transverse colon cancer with gastrocolic fistula and synchronous rectal cancer, a one-stage curative operation was performed. Pathologically, both cancers were well-differentiated adenocarcinomas, but the transvers colon cancer was partially mucinous. Lymph node metastasis was absent. Gastrocolic fistula complicating colon cancer is rare, to our knowledge, with only 29 cases reported in Japan. A curative operation was performed in 73%of these cases, including ours, and lymph node metastasis was observed in only 22%. This suggests that colon cancer with a gastrocolic fistula might undergo less lymph node metastasis despite increased invasion depth, and that a curative operation for the colon cancer and gastrocolic fistula should be considered.