The herniation of small bowel through Winslow's foramen is a rare type of internal hernia which can cause ileus; however, a hernia traversing the lesser sac is even more unusual. To the best of our knowledge, only 25 cases of herniation through Winslow's foramen and 10 cases of lesser sac hernia have been reported in the Japanese literature. We describe herein the case of a 33-year-old man who presented to our hospital complaining of abdominal pain in whom a plain abdominal radiograph revealed small bowel gas with air-fluid levels, suggesting ileus. Following admission, an ileus tube was inserted, but the intestinal shadow did not improve and surgery was performed based on suspicion of an internal hernia. Approximately 100 cm of ileum was found to have herniated through a defect in the lesser omentum after passing through Winslow's foramen. Since the herniated bowel was viable, manual reduction without resection was performed. The patient had a satisfactory postoperative course, and was discharged on postoperative day 11. There are many unknown aspects surrounding the etiology of Winslow's foramen hernia and lesser sac hernia, and although internal hernia is a rare cause of ileus, its possibility should be kept in mind.