A 67-year-old man transferred to our hospital with abdominal pain was diagnosed as having colon ileus caused by sigmoid colon cancer. After reducing the colon pressure with an ileus tube, he received a laparoscopic-assisted sigmoidectomy. On and after the eighth day after the surgery, unusual bleeding (melena, hemoperitoneum, muscle hemorrhage, and subcutaneous hemorrhage) was noted. Coagulation tests revealed exclusively prolonged APTT, normal PT, a decrease in the factor VIII, and a high titer of factor VIII inhibitor. Acquired hemophilia A was thus diagnosed. We started the therapy with factor VIII concentrates and predonizoron. On the next day, he was transferred to another hospital for hematological treatments. Thereafter administration of steroids alone and cyclosporine alone were unsuccessful and the combined therapy with cyclophosphamide and steroids achieved a partial remission. The patient was saved. Acquired hemophilia A is a rare and fatal condition of coagulopathy. When we surgens encounter a sudden onset of severe hemorrhagic tendency of unknown origin, we should consider a possibility of acquired hemophilia A.