Congenital mesocolic hernia is a rare disease which resulted from abnormal rotation of midgut during development period. It is classified by the site of mesocolic defect and hernia sac. The transverse mesocolic hernia is extremely rare. A 57-years-old man was admitted due to nausea, epigastric pain and abdominal distension. He had been diagnosed as duodenal ulcer with pyloric deformity 18 months ago. Simple abdomen X-ray with gastrografin swallowing and abdominal CT highly suggested internal hernia. Surgical exploration was done under the diagnosis of internal hernia. There was a 3 cm sized defect at transverse mesocolon and nearly total small intestine, which was herniated into the lesser sac and retroperitoneal space without strangulation. After simple resection of transverse mesocolic defect and transverse colon, reduction of herniated bowel and simple closure of transverse colon were done.