We report the case of a patient with paraduodenal hernia diagnosed incidentally during an operation for transverse colon cancer. The patient was a 77-year-old woman who complained of dizziness. Laboratory data revealed no abnormal findings except slight anemia. Barium enema and colonoscopic examination revealed an irregular surfaced mass, about 5.0 cm in size, located near the flexure of the spleen of the transverse colon. A biopsy of the mass was performed, and a moderately differentiated adenocarcinoma was diagnosed. In April 2009, following the diagnosis of transverse colon cancer, laparotomy was performed, which revealed that a few loops of the jejunum were herniated through the orifice into the space posterior to the transverse mesocolon. Moreover, the jejunal loops were located right between a shifted left branch of the middle colic artery and ascending left colic artery. There were no ischemic changes in the jejunum. These findings were consistent with a left paraduodenal hernia associated with transverse colon cancer. The scheduled left hemicolectomy was performed in addition to a radical operation of the left paraduodenal hernia. The abdominal computed tomography (CT) images were reviewed postoperatively. The scan projection radiogram obtained by CT revealed a packing of jejunal loops in the middle of the abdomen. Abdominal CT revealed ascending left colic artery at the left edge of a packing of jejunal loops. The patient was discharged from our hospital 14 days after the surgery without any complications. Left paraduodenal hernias are rare and constitute less than 0.4% of all intestinal obstructions. Retrospectively reviewed, the preoperative CT is suggestive. In addition to the packing of jejunal loops in the middle of the abdomen, ascending left colic artery was clearly observed at the left edge of the packing of jejunal loops, which indicates left paraduodenal hernia.