A 65-year-old man who was diagnosed with left inguinal hernia 15 years earlier but had let it alone presented to our department with the hope of undergoing surgery for the gradually enlarging swelling. In accordance with his hope, we performed colonoscopy before surgery. During colonoscopy, we could not insert nor remove the endoscope, and at the same time, his lower abdominal pain was exacerbated. A hard hemisphere bulge was identified in the left inguinal region. An emergency CT scan revealed the sigmoid colon and the endoscope to have been incarcerated in the hernia sac. Immediate attempt of manual reduction was unsuccessful, so that we decided to perform emergency operation. Incision of the hernia sac via an anterior approach under general anesthesia showed only small quantity of bloody ascites, but ischemic change was absent in the wall of the sigmoid colon. We found a detrition in part of serosa surface and restored it. After we gently removed the endoscope which had been trans-anally inserted, we reduced the sigmoid colon into the abdominal cavity and repaired the hernia by the Direct Kugel method. Although incarceration of a colonoscope is a rare accident, it might be important to keep in mind as a possible complication of examination for hernia patients.