The purpose of this study is to evaluate the the efficacy of TA-650 using Clinical activity index (CAI) score and other evaluation indicators in pediatric patients with moderate to severe ulcerative colitis after TA-650 administration of at a dose of 5 mg/kg at weeks 0, 2, and 6, and then every 8 weeks at weeks 14 and 22. The safety and pharmacokinetics are also evaluated.