A 58-year-old female was referred to our hospital for endoscopic gatric polypectomy in January, 2002. Endoscopic examination revealed Yamada type IV polyp, 4.5cm in length, on the posterior wall of the middle gastric body. Histological findings of the polypectomied specimen suggested a hyperplastic polyp with cells of papillary adenocarcinoma and signet ring cell carcinoma, respectively, in the polyp. The phenotype of the carcinoma in the polyp was evaluated by immunohistochemicaly with CD10, MUC2 and human gastric mucin (HGM) . The carcinoma and the hyperplastic foveolar epithelium were only positive for HGM. Therefore the carcinoma was classified as gastric type. It is said that this type changed more commonly to undifferentiated type during the course of invasion of the mucosa and/or the submucosa or deeper. This case was thought to be very rare in that signet ring cell carcinoma and papillary adenocarcinoma were developing in a hyperplastic polyp in the stomach.