Diarrhea, melena, and lower abdominal pain developed in a male in his 20s and colonoscopy showed pancolitis-type severe ulcerative colitis (UC). Treatment was initiated with 4,000 mg of 5-aminosalicylic acid and 60 mg/day of prednisolone, but the symptoms and inflammatory reaction worsened with prednisolone dose reduction. Tacrolimus was added to the treatment, which subsequently induced remission. Serial colonoscopies during the treatment showed improvement in ulcer and mucosal edema throughout the entire large intestine, but a new solitary round ulcer appeared at the end of the ileum. Since no signs of Behçet's disease were noted, it was considered as a simple ulcer, a complication of UC. Tacrolimus treatment was continued based on continued improvement in clinical features and colonic mucosa, excluding the end of the ileum. Colonoscopy at 6 months after initiation of tacrolimus showed healing of the large intestinal mucosa, although mild congestion was still noted. The solitary round ulcer at the end of the ileum improved to a small erosion. We report the improvement of a simple ulcer that developed during tacrolimus treatment.