A 40's man was referred to our hospital for the investigation of fever of unknown origin lasting for a month. The laboratory data showed a prominent inflammatory reaction and a high titer of PR3-ANCA. Despite the various imaging studies and bacteriological examinations, the cause of the fever could not be detected until he complained of abdominal pain with bloody stool that appeared during hospitalization and which prompted colonoscopy, resulting in the diagnosis of moderate ulcerative colitis of the descending colon. Although temporal improvement was achieved by mesalazine administration, the symptom exacerbated again. Then, a combination of steroid administration and the leukocytapheresis (LCAP) was performed, but it was also not effective. His rapidly deteriorating condition with the lesion extending to whole colon necessitated subtotal colectomy. He has been afebrile and in good condition since the operation, which indicates the cause of the fever was due to ulcerative colitis.