Guidelines recommend surveillance colonoscopy for patients with an ulcerative colitis (UC) duration of 8-10 years. We experienced a patient who had not undergone UC surveillance. A 35-year-old Japanese woman developed diarrhea and abdominal pain in January 2018 and was diagnosed with UC. She underwent medical therapy, and 18 months after onset of UC colonoscopy indicated that her UC activity was remission and showed no cancer lesions. Twenty-four months after onset, colonoscopy revealed a tumor in the ascending colon, and the biopsy revealed tubular adenocarcinoma. She had no family history of colorectal cancer. There were no findings of distant metastases or primary sclerosing cholangitis. Laparoscopy-assisted anus-preserving total proctocolectomy, the creation of a J-type ileal pouch, ileal pouch anal anastomosis, and the creation of an ileostomy were performed. The pathological report was type 3, 30 × 27-mm, adenocarcinoma (por2 > tub2), pT4a, Ly1a, V1a, budding grade 3, pN0, M0, Stage IIb. Some colitic cancers such as our patient’s may not conform to the existing guidelines. When a colonoscopy is being performed for a UC patient, even if its timing is less < 8 years since the UC onset, suspicious lesions should be biopsied considering the possibility of cancer.