Adenocarcinoma is a common histological type of ulcerative colitis-associated cancer (UCAC), whereas neuroendocrine carcinoma (NEC) is extremely rare. UCAC is generally diagnosed at an advanced stage, even with regular surveillance colonoscopy. A 41-year-old man with a 17-year history of UC began receiving surveillance colonoscopy at the age of 37 years; 2 years later, dysplasia was detected in the sigmoid colon, and he underwent colonoscopy every 3 to 6 months. Approximately 1.5 years thereafter, a flat adenocarcinoma lesion occurred in the rectum. Flat lesions with high-grade dysplasia were found in the sigmoid colon and surrounding area. The patient underwent laparoscopic total proctocolectomy and ileal pouch-anal anastomosis with ileostomy. Adenocarcinoma was diagnosed in the sigmoid colon and NEC in the rectum. One year postoperation, recurrence or metastasis was not evident. Regular surveillance colonoscopy is important in patients with long-term UC. A histological examination of UCAC might demonstrate NEC.