A case of a Richter hernia at the insertion site of the drainage tube following open abdominal surgery is reported. A 54-year-old man underwent an emergency operation for an idiopathic rectal perforation. A partial resection of the rectum and drainage using four 10-mm (outer diameter) drainage tubes with round cross sections was performed. Despite an uneventful early postoperative course, an emergency reoperation was required for peritonitis due to a bowel perforation 14 days after removing the drain inserted into the rectosacral space. A laparotomy revealed an incarcerated Richter hernia with ileal perforation through the 10-mm drainage site. The postoperative course after a partial resection of the ileum and drainage with Penrose drains was uneventful. This is the first report of a Richter hernia through the insertion site of a drainage tube in abdominal surgery. The possible occurrence of a Richter hernia in cases with postoperative drainage using large-size round drainage tubes should thus be considered in such patients.