A 49-year-old man was admitted for abdominal pain and vomiting. Abdominal distention and tenderness was observed, but muscular defence was not remarkable. Plain X-ray of the abdomen showed multiple air-fluid levels in the dilated small intestine, and an abdominal computed tomography (CT) showed an extended intestinal tract and ascites. Strangulated ileus could not be ruled out and an emergency operation was conducted. Upon laparotomy, we found moderate amount of chylous ascites, and the small intestine was twisted and rotated about 360° clockwise by mesenteric torsion. The necrosis of small bowel was not present, so reduction of torsion was performed. This case did not revealed anatomical abnormalities nor malformation, and the patient was diagnosed as primary small bowel volvulus. It is rare, and 47 cases, including ours, have been reported in Japan. For diagnosis, abdominal CT is useful. It is also reported. Small intestine may develop rapid necrosis and can go into shock and cautions are required in not missing the timing of surgery.