Chylous ascites occurring after abdominal surgery is rare. Despite being potentially critical, there is no definite treatment guideline because of its rarity. Here we present a case of massive chylous ascites occurring after rectal surgery which was successfully treated with an oral fat-free elemental diet (ED). A 67-year-old man underwent low anterior resection with para-aortic lymphadenectomy for advanced rectal cancer. Early postoperative course was uneventful and the patient was discharged from hospital 10 days after surgery; however, after discharge, abdominal distension rapidly developed. Abdominal computed tomography (CT) performed 3 weeks after surgery revealed massive ascites and laboratory findings showed remarkable hypoproteinemia and lymphopenia. Urgent diagnostic paracentesis showed the ascites to be a white milky fluid containing high levels of triglycerides (564 mg/dl), leading to a diagnosis of chyloperitoneum. Daily nutrition of the patient was entirely with a fat-free ED (30 kcal/kg/day of Elental®, Ajinomoto Pharmaceutical Co. Ltd, Tokyo, Japan). After the initiation of oral Elental®, abdominal distension, hypoproteinemia, and lymphopenia gradually improved. Abdominal CT performed 7 weeks after surgery showed no ascitic fluid in the abdomen, and thereafter a normal diet was initiated. Since then, no relapse of chyloperitoneum has been proven. As a result, the chylous ascites was successfully treated in the outpatient clinic.