The purpose of this study is to verify the safety and efficacy of OPC-6535 and determine the optimal dose by once-daily oral administration of OPC-6535 at 25 or 50 mg or placebo for 8 weeks in combination with base treatment (either a fixed oral dose of 5-aminosalicylic acid [5-ASA] or a fixed oral dose of 5-ASA plus enteral nutrition) in 180 patients with active Crohn's disease.