To establish the efficacy of 40 mg/day doses of istradefylline for the change in Unified Parkinson's Disease Rating Scale (UPDRS) part-III (Motor examination) score in patients with Parkinson's disease (PD).

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.

Inclusion Criteria:

1. UK Parkinson's Disease Society (UKPDS) brain bank criteria (Step 1 and 2) for PD.
2. PD stages 1-3 for Modified Hoehn and Yahr Scale.
3. Be at least 30 years of age.

Exclusion Criteria:

1. Neurosurgical treatment for PD.
2. History of psychosis.
3. Diagnosis of cancer within 5 years.
4. Diagnosis of clinically significant illness of any organ system.
5. Mini-mental status examination score of 25 or less.
6. Taking any excluded medications.
7. History of drug or alcohol abuse or dependence within the past two years.
8. History of seizures or neurological malignant syndrome.
9. Clinical depression.
10. Pregnant or lactating females.