A 11-year-old boy was admitted to the hospital because of an examination of an intra-abdominal tumor which was detected by ultrasonography at elsewhere. The patient was pointed out proteinuria at a school medical checkup, but he was asymptomatic. The abdomen was flat and soft, and no tumor was palpated. An abdominal echogram and computed tomography revealed a well-defined cystic lesion between the pancreas and stomach. At the operation, the cystic mass was lacated above the upper border of pancreas (body portion) and connected with the posterior wall of cardia. The tumor was resected completely with a part of the stomach. The resected tumor was 6.5×5.5×5.5cm in size and ulcers were found on the internal surface. Microscopically, there was a common muscular layer between the cyst and stomach, and the wall was lined with two different epitheria-mainly the stratified ciliated columnar epitherium and the immature fundus epitherium. The definite diagnosis of gastric duplication was made. The microscopic findings suggest that the abnormal deviation of the primitive foregut is one of the cause of the gastric duplication.