Idiopathic sclerosing peritonitis is a rare disease characterized by fibrosis and adhesion of the peritoneum to loops of the small intestine. It may be the cause of an unusual surgical emergency such as small bowel ileus. It is diagnosed predominantly in female adolescents. We report the case of an idiopathic sclerosing peritonitis in Korea. A 38-year-old man visited emergency room for recurrent small bowel ileus and migrating mass like lesion. Computed tomography (CT) of abdomen showed acute peritonitis with a diffuse wall thickening of terminal ileum and extraluminal fluid collectionaround the terminal ileum. He underwent laparotomy. The ileocolectomy with adhesiolysis was performed and its pathological examination revealed the characteristic findings of idiopathic sclerosing peritonitis. Symptoms recurred 2 months after surgery, but improved with steroid treatment.