A 77-year-old woman presented with a chief complaint of bloody stools. Detailed examination revealed a semi-circumferential type 2 tumor in the lower rectum, and a diagnosis of Group 5, tub1-2, cT3N2aM0, cStage Ⅲb rectal cancer was made. Preoperative abdominal CT scans revealed a shunt in the inferior mesenteric vein and left ovarian vein. Laparoscopic Hartmann's procedure was performed, and when the sigmoid mesentery was moved from the inner side, a shunt flowing from the left ovarian vein to the inferior mesenteric vein in the sigmoid mesentery was found, which was then dissected. The operating time was 253 min, and blood loss was approximately 140 g. There was no postoperative liver dysfunction, and the patient was transferred to another hospital on postoperative day 36. Causes of portal-systemic shunts are portal hypertension occurring due to liver cirrhosis or congenital causes and organ adhesion from abdominal surgery. In this case, there was no liver cirrhosis, and the blockage of the left renal vein perfusion by the superior mesenteric artery may have resulted in congestion and varicose of the left ovarian vein. Furthermore, the shunt with the inferior mesenteric vein may have been formed due to the adhesion of the left ovarian vein after ovariectomy. If preoperative tests reveal varices, a surgical treatment is recommended while keeping in mind the possibility of shunt formation as in this case.