A Phase II clinical trial has been initiated to evaluate the efficacy and safety of endoscopic submucosal dissection for intramucosal (cT1a) gastric cancer of undifferentiated type. Patients with cT1a gastric cancer with undifferentiated-type adenocarcinoma are eligible for the study. The tumor size should be 2 cm or less without ulceration. The study will enroll a total of 325 patients from 51 institutions over a 4-year period. The primary endpoint is proportion of 5-year overall survival (% 5-year overall survival) in patients with undifferentiated dominant type. The secondary endpoints are overall survival, relapse-free survival, distant metastasis-free survival, % 5-year overall survival without either recurrence or gastrectomy, % en-bloc resection with endoscopic submucosal dissection, % pathological curative resection with endoscopic submucosal dissection, % 5-year overall survival in patients with differentiated dominant type, % 5-year overall survival in patients with pathologically curative resection with endoscopic submucosal dissection and adverse events.