<Period I (double-blind period)> A multicenter, collaborative, double-blind, parallel comparative study is conducted using IBS-C patients as subjects to verify efficacy of ASP0456 and examine the safety. After the 2-week bowel habit observation period, subjects satisfying the primary enrollment criteria are randomly allocated to either ASP0456 group or placebo group, and orally administered the drug or placebo once daily before breakfast for 12 weeks.