A 66-year-old woman was admitted to our hospital with the chief complaint of heart burn in September 2007. Endoscopic examination revealed a submucosal tumorous lesion which maintained the mucosal pattern without central depression on the posterior wall of the upper gastric body, measuring about 3cm in diameter. A biopsy of the elevation was reported to be GroupII. Endoscopic ultrasonography showed a homogeneous, hypo echoic mass localized in the submucosal to subserosal layer of the gastric wall. Laparoscopic partial resection of the stomach was performed under a diagnosis of submucosal tumor. Histologically the tumor was diagnosed as adenocarcinoma (tub1>por2, ss, ly2, v2, margin-). Extremely well-differentiated adenocarcinoma (EWDA) cells in mucosa had grown through the muscularis mucosae into the subserosa and transformed into poorly differentiated type. After ruling out possibilities of metastasis and invasion from other organ cancer, we scheduled total gastrectomy as the operation for advanced gastric cancer after the partial resection, but were forced to perform exploratory laparotomy because of peritoneal dissemination. Reports of EWDA resembling submucosal tumor in morphology are rare. We should be careful in treatment strategy because of difficulty in making the histological diagnosis of carcinoma on biopsy.