To establish the efficacy of 40mg/d doses of istradefylline for the change in UPDRS part-III (Motor examination) score in patients with Parkinson's disease (PD). Patients who meet entry criteria will be randomly assigned to receive study drug in 1 of 2 arms: Condition A (doses of 40mg/d istradefylline) in Period 1 followed by Condition B (matching placebo) in Period 2 (putting 4 weeks interval phase between two periods ) or Condition B in Period 1 followed by Condition A in Period 2(putting 4 weeks interval phase between two periods ). Patients will be participated for 12 weeks and will have interim visits and end of treatment visit to assess the efficacy and safety of istradefylline.