A 52-year-old woman had a surgical history of breast cancer. A follow-up blood test showed that the level of carcinoembryonic antigen was elevated. Esophagogastroduodenoscopy revealed a Borrmann type IV tumor in the stomach, and biopsy revealed poorly differentiated adenocarcinoma with signet ring cells. The preoperative diagnosis was gastric cancer, and gastrectomy was attempted, but the procedure was changed to laparotomy because of the peritoneal dissemination of cancer. Chemotherapy for gastric cancer after laparotomy enabled the patient to maintain stable disease without progression for a long period. Thirteen years later, poorly differentiated adenocarcinoma of the stomach, which had not been detected for 10 years despite routine examination every year, was identified by endoscopic biopsy. Immunohistochemistry studies showed strong positivity of both estrogen and progesterone receptors of adenocarcinoma. We reviewed all the past pathological specimens and finally corrected the diagnosis as a gastric metastasis of breast cancer. Even if primary gastric cancer is strongly suspected, histopathological examination is recommended to check whether the tumor is instead a gastric metastasis.