A 59-year-old man was referred in 2007 to our hospital with duodenal adenoma. Esophagogastroduodenoscopy (EGD) showed a slightly elevated whitish lesion at the 2nd portion of the duodenum. The lesion was 15 mm in size and appeared to be accompanied by a scar. Biopsy showed low grade adenoma and hence the lesion was monitored. On endoscopic examination, it gradually changed to a reddish color and became more depressed. In 2012, the lesion was examined using a magnifying endoscope with narrow band imaging (ME-NBI) . This revealed an irregular microvascular pattern of the lesion. At this stage the lesion was diagnosed as a cancer, and endoscopic biopsy of this identified well differentiated adenocarcinoma (tub1) . ESD was performed and the resected specimen showed D, 15 × 10 mm, Type 0-IIa, tub1, pT1a, ly (−) , v (−) , pHM0, pVM0. In this case, ME-NBI was useful to diagnose the cancer. The endoscopy was performed every one year. Repeated biopsy procedures result in fibrosis of the submucosal layer of the duodenum and may make subsequent ESD difficult. Consequently, we recommend the use of ME-NBI as optical biopsy for duodenal adenoma.