The patient was a 73-year-old woman with hematemesis, who was brought to our hospital by an ambulance. Emergency upper gastrointestinal endoscopy was performed, and a thickened wall and multiple ulcers were noted in the middle- lower region of the stomach body. The biopsy revealed gastric cancer (por), and the macroscopic depth of invasion was up to the subserosal layer. Multiple white granular protruding lesions were observed in the duodenal bulb and were diagnosed as follicular lymphoma by biopsy. As follicular lymphoma generally takes a long time to progress in many cases, the vital prognosis would be determined by the gastric cancer. Thus, total gastrectomy was performed for gastric cancer. For reconstruction, the double-tract method was used. Duodenal follicular lymphoma was continuously monitored by upper gastrointestinal endoscopy. Here, we describe the case of a patient with a complication of follicular lymphoma arising from duodenal and gastric cancer, which is very rare, and report this case along with a literature review.