Inclusion Criteria:

1. Man or woman, aged 18 to 65 years, outpatient
2. Primary OCD patients diagnosed with DSM-IV-TR, having OCD symptoms for more than six months
3. Severity: Y-BOCS score of >= 20 at screening and baseline
4. No history or evidences of clinically significant physical problem, or abnormal laboratory findings at screening
5. Drug free period of at least 2 weeks before study drug administration (in case of fluoxetine, at least 5 weeks). However, allowed medications in this study could be taken even before study drug administration.

Exclusion Criteria:

1. primary active DSM-IV axis I diagnosis other than OCD
2. History of substance, including alcohol, dependence and psychotic symptoms
3. Current depression A. Risk of suicidal attempt B. 20-item Hamilton Depression Rating Scale (HAM-D)(exclude 21th item, obsession) of >17 at screening or baseline
4. Drug allergy A. Known or suspected hypersensitivity to escitalopram or citalopram B. History of severe drug allergic reactions or hypersensitivity
5. History of no response to escitalopram or citalopram treatment
6. History of electroconvulsive therapy or received an injectable antipsychotic formulation within 6 months before screening
7. Women who are pregnant, planning to become pregnant, or breast-feeding
8. Ongoing cognitive behavior therapy (CBT) of OCD
9. Hoarding or collecting type
10. Unable to perform study protocol as clinically assessed by the investigator (ex, severe personality disorder)