Inflammatory bowel disease (IBD) is regarded as one of the risk factors for colorectal cancer, and early detection of cancer in these patients may be difficult, especially in pediatric patients. Prognosis of pediatric colorectal cancer is known to be poor, because of delayed diagnosis and unfavorable differentiation. We report a case of a pediatric patient with a 10-year history of ulcerative colitis who was diagnosed with sigmoid colon cancer when he was 15 years old. He underwent proctocolectomy with ileal pouch anal anastomosis. Postoperative pathological examination of the tumor revealed adenocarcinoma. The pericolic tissue layer was infiltrated, but metastases were not found in either of the two lymph nodes. Children with a long history of predisposing factors such as IBD need particular attention to the possibility of colorectal cancer. Early diagnosis through regular screening with colonoscopy is one of the most important critical factors for a good prognosis. Inflammatory bowel disease (IBD) is as one of the risk factors for colorectal cancer, and early detection of cancer in these patients may be difficult, especially in pediatric patients. Prognosis of pediatric colorectal cancer is known to be poor, because of delayed diagnosis and unfavorable differentiation. We report a case of a pediatric patient with a 10-year history of ulcerative colitis who was diagnosed with sigmoid colon cancer when he was 15 years old. He underwent proctocolectomy with ileal pouch anal anastomosis. Postoperative pathological examination of the tumor The pericolic tissue layer was not found in but of metastases were not found in either of the two lymph nodes. children with a long history of predisposing factors such as IBD need particular attention to the possibility of colorectal cancer. Early diagnosis through regular screening with colonoscopy is one of the most important critical factors for a good prognosis.