A 78-year-old man underwent distal partial gastrectomy and Billroth I reconstruction for early gastric cancer in May 1995, then was followed up by us. In March 2005, he was found in upper gastrointestinal endoscopy to have an elevated Isp lesion of the duodenum. Histological examination of biopsy specimens showed on inflammatory polyp and no malignancy. After one year of observation, the lesion became a IIa+IIc lesion in upper gastrointestinal endoscopy. Biopsy specimens showed group V and suspected signet-ring cell carcinoma. After further examination, we conducted partial resection of the duodenum. Macroscopically, the 8×6 mm tumor appeared to be a IIa+IIc lesion located in the second portion of the duodenum. The histological diagnosis was signet-ring cell carcinoma, within the membrane, ly0 and v0. The surgical margin was negative. We conducted curative surgery. He is alive without recurrence almost 10years after the operation. Billroth I reconstruction enabled us to diagnose duodenal cancer early in this case.