A Japanese man in his 60s presented with complaints of epigastric pain and weight loss. A gastrointestinal endoscopy revealed multiple gastric ulcers and an irregular mound located on the wall of the lower gastric body along the greater curvature, which was suspected to be cancerous. A biopsy revealed that it was a Group 2 tumor even though the biopsy was repeated 4 times. He was referred to our hospital and 3 biopsies were performed. The final result classified the tumor as Group 4. The patient underwent surgery and the pathological examination revealed an extremely well-differentiated adenocarcinoma( EWDA). An EWDA is characterized by a well-formed mucinous gland with little or no nuclear atypia, which makes preoperative biopsy diagnosis difficult.