A man in his seventies was referred to our hospital for radical therapy for advanced rectal cancer with multiple liver metastases. A colonic stent had already been placed in his rectum at the previous hospital because of malignant colorectal obstruction, so our therapeutic strategy was to perform systematic chemotherapy after resection of the primary tumor. Laparoscopic low anterior resection with a covering stoma was performed under general anesthesia. At about one hour after the surgery, the patient had sudden abdominal pain with watery diarrhea, and a similar discharge from his drainage tube. We suspected peritonitis caused by bowel perforation and emergency surgery was performed. The operative findings showed that his peritonitis was caused by anastomotic leakage from the rectum. Radical lavage of the abdominal space and reconstruction of colostomy was performed. The patient gradually recovered and we were able to start systematic chemotherapy at one month after the surgery. Anastomotic leakage immediately after anterior resection caused by watery diarrhea is rare, and it may be concerned with several issues. The covering stoma is intended to stop anastomotic leakage but it cannot prevent all cases of leakage especially when obstruction is present. We recommend that preventive measures be taken against anastomotic leakage, including intraoperative leakage tests or anal decompression tube placement.