Main Inclusion Criteria:  Males and females adults aged 20 to 65 years Diagnosed as schizophrenia prior to the screening visit On treatment with a stable dose of clozapine 200 mg/day for at least 12 weeks before the screening visit and taken at 100 mg twice daily during the study period  Main Exclusion Criteria:  Subjects who have a medical history specified in protocol Subjects with confirmed abnormal laboratory values specified in protocol Subjects who have a medication history or safety risks specified in protocol Subjects who can not comply with requirements of pharmacokinetic sampling as per protocol Subjects who are expected to have the prohibited concomitant medication therapy during the study period Pregnant women or breast-feeding women or men and women who has possibility of pregnancy Subjects who are not suitable for the clinical trial