We report a case of severe ulcerative colitis complicated with primary diabetes mellitus that was effectively treated with oral tacrolimus. A 36-year-old man suffered his first attack of ulcerative colitis. Because it was difficult to control the patient's diabetes, we instituted tacrolimus and azathioprine without a corticosteroid regimen as the initial treatment. Within two weeks, the ulcerative colitis activity went into remission and the patient's diabetes did not worsen.