Inclusion criteria:

• Target disease: Patients diagnosed as having the following primary disease and current major depressive episode for at least 8 weeks on the basis of DSM-IV-TR criteria.

  • Major Depressive Disorder, Single Episode (296.2x) (excluding those with psychotic features)
  • Major Depressive Disorder, Recurrent (296.3x). (excluding those with psychotic features)
• HAM-D (17 items) total score >/=18.
• Age: >/=65 years old (at the time of informed consent)
• Gender: Male or female.
• Inpatient or outpatient status: Either
• Informed consent: The subject himself/herself must give written informed consent.

Exclusion criteria:

• Patients with predisposition to seizure (who currently have or have a past history of seizure, febrile convulsive seizure in infancy, cerebral tumour, cerebrovascular disorder or head injury, who have a family history of idiopathic seizure, patients with diabetes who have been treated with oral hypoglycaemics or insulin, or who use drugs lowering the threshold of seizure)

• Patients who currently have or have a past history of the following disorders:

  • Anorexia nervosa (DSM-IV-TR 307.1)
  • Bulimia nervosa (DSM-IV-TR 307.51)
• Patients with a history of manic episode
• Patients with a past or current DSM- IV-TR diagnosis of schizophrenia or other psychotic disorder
• Patients with a current DSM-IV-TR Axis II diagnosis (e.g., antisocial or borderline personality disorder)
• Patients starting psychotherapy (except for supportive psychotherapy not aimed at therapeutic efficacy and unlikely to affect efficacy evaluation) and standardized cognitive behaviour therapy within 12 weeks prior to the start of the treatment phase
• Patients with a diagnosis of substance abuse (alcohol or drug) by the DSM-IV-TR criteria or with a diagnosis of substance dependence within 1 year prior to the start of the treatment phase
• Patients who have received electroconvulsive therapy within 24 weeks prior to the start of the treatment phase
• Patients who have taken MAO inhibitors (selegiline hydrochloride) within 2 weeks prior to the start of the treatment phase
• Patients who have taken another investigational drug within 12 weeks prior to the start of the pre-treatment phase
• Patients who have attempted suicide within 24 weeks prior to the start of the treatment phase, or patients for whom the score of the suicide-related item of HAM-D is >/=3, or patients in whom the risk of suicide is judged to be high by the investigator (sub-investigator).
• Patients in whom the risk of homicide is judged to be high by the investigator (sub-investigator).
• Patients with a history of hypersensitivity to 323U66
• Patients with serious cerebral disease
• Patients who have ECG or clinical evidence of any cardiac condition that the investigator (sub-investigator) assesses the subject is predisposed to ischemia or arrhythmia
• Patients with serious physical symptoms (i.e. cardiac/hepatic/renal disorder, hematopoietic disorder).

The index of seriousness is Grade 3 of "Criteria for classification of seriousness of adverse drug reactions to pharmaceutical products, etc." (PAB/PSD No.80 in 1992).

• Patients who have a history or complicated carcinoma or malignant tumour.
• Patients whose major depressive disorder is due to direct physiological effects of a general medical condition (for example, hypothyroidism, Parkinson's disease, chronic pain)
• Patients with systolic blood pressure of >/=160 mmHg or diastolic blood pressure of >/=100 mmHg at the start of the Treatment Period
• Patients diagnosed with dementia
• Patients who are inappropriate for participating in the study in the judgement of the investigator (sub-investigator).