Disclosed is a therapeutic agent for ulcerative colitis, which comprises mizoribine, has a therapeutic effect on ulcerative colitis, does not have concerns about the occurrence of an adverse side effect during the maintenance of remission of ulcerative colitis, and can be administered for a long period. Specifically disclosed is a therapeutic agent for ulcerative colitis, which comprises mizoribine. The therapeutic agent can be used for an ulcerative colitis patient who has received a leukocytapheresis therapy and is not administered with any immunosuppression agent other than mizoribine at least after the leukocytapheresis therapy is started. The administration of therapeutic agent to the patient is started at a point of time which lies during the application of the leukocytapheresis therapy and after the clinical condition of the patient is ameliorated. The therapeutic agent is administered to the patient at least once a day so that the maximum blood mizoribine level in the patient becomes not lower than 0.6 µg/ml and lower than 1.5 µg/ml.
