A 46-year-old woman was transferred to our facility with the complaint of epigastric discomfort of more than four months duration. She had undergone partial gastrectomy with Billroth I anastomosis 19 years prior to the admission. An upper GI series revealed multiple, variously sized elevated lesions scattered over the remnant stomach. Fiberscopic examination showed the gastric mucosa replaced by several elevated lesions and giant fold embellished by erosion, injection, and necrotic debris. Pathological examination revealed malignant lymphoma of the large cell type whose main growth was in the mucosa and submucosa. This case was significant in that malignant lymphoma occurred in the remnant stomach and that the tumor converted M-protein in the serum, which was confirmed as IgM-k in the cytoplasma immunohistochemically.