A case of primary sall bowel volvulus presenting with diffuse peritonitis is reported. A 54-year-old man was admitted to a near hospital with the complaint of abdominal pain, and palliatively treated at the hospital. But his symptoms were not improved, so he was refered to our hospital. On admission the abdomen was distended and muscular defense and Blumberg sign were noted. Abdominal X-ray film showed a small bowel gas shadow with niveau. Emergency operation was carried out because his symptoms suggested diffuse peritonitis. On laparotomy, massive small bowel necrosis was found and the root of the small intestinal mesentery was twisted clockwise about 360°. Resection of the length of the survived intestine was 1.2 m. We diagnoted as primary small bowel volvulus because the bands and adhesion were not noted in the abdominal cavity. The patient was treated with TPN and ED after operation and was discharged on the 50th day. Primary sall bowel volvulus rarely occurs, but demands careful attention in its possible severe progress.