We discuss a rare complication in a patient who underwent laparoscopic colectomy. A 69-year-old woman underwent laparoscopy-assisted right colectomy for cancer of the ascending colon. Two months after the operation, bowel obstruction developed. Decompression with a long intestinal tube failed to resolve the obstruction. Thus, surgery was performed. Abdominal exploration revealed a strangulated ileal loop caused by herniation through the mesenteric opening at the anastomotic site. The mesenterium had not been sutured during the previous operation. The anastomotic segment had twisted semicircularly and adhered to the retroperitoneum, so the mesenteric opening had narrowed.