A 58-year-old woman underwent laparoscopy-assisted transverse colectomy for transverse colon cancer. On postoperative day 7, she experienced sudden abdominal pain accompanied by vomiting and fever. Computed tomography showed a small bowel obstruction caused by an internal hernia. Laparotomy revealed an internal hernia through the mesenteric defect at the anastomotic colonic stumps, which had not been closed in the previous operation. Almost the entire small bowel protruding through the mesenteric defect was found in the omental bursa. We resected part of the jejunal loop, which was strangulated and congested by an adherent band. Our experience suggests that if the mesenteric defect is relatively small, it should be closed completely during laparoscopy-assisted colectomy; however, more studies are required to determine the indications for closure of the mesenteric defect to prevent this complication.