We report a case of primary gastric carcinoma with a macroscopic appearance indistinguishable from that of a submucosal tumor. A 48-year-old man visited our hospital with a chief complaint of epigastric discomfort. Endoscopic examination revealed a protruding lesion with a well defined margin on the anterior wall of the gastric antrum. Most of the tumor surface was covered with apparently normal gastric mucosa and a shallow recess with mild erosion was observed on the top. Abdominal ultrasonography showed a hypoechoic lesion with an irregular margin under the gastric mucosa. Laboratory examination revealed an elevated CA19-9 level of 106.9 U/ml. In spite of repeated bouling biopsies, no histological diagnosis could be obtained before surgery. However, gastrectomy with regional lymph node dissection was performed because of the high likelihood of gastric cancer, in view of the markedly elevated CA19-9 level and irregular tumor margin demonstrated by abdominal ultrasonography. The tumor was diagnosed histologically as papillo-tubular adenocarcinoma invasive to the serosa with marked vessel infiltration. No metastasis was found in the regional lymph nodes. Gastric cancer resembling submucosal tumor is rare and often difficult to diagnose. Careful estimation of the possibility of gastric cancer and the informed consent of the patient are critically important, in cases of suspected primary gastric cancer resembling submucosal tumor, in order to decide the form of treatment.