We herein report what, to our knowledge, is only the fourth known case of segmental dilatation of the duodenum. Antenatal ultrasonography revealed an intraabdominal cyst in the fetus, but the exact location of the segmental dilatation was difficult to find preoperatively. Moreover, even using computed tomography, it was not possible to make a diagnosis prior to surgery. The anatomic characteristics of duodenal dilatation made it difficult to perform the usual resection techniques. In fact, the surgical procedure was different from the previously reported cases. We performed a partial resection of the duodenum followed by a tapering procedure to preserve the ampulla of Vater. The infant had an uneventful postoperative course, and sufficient growth and development has been achieved.