To examine the safety and efficacy of long term administration of aripiprazole as an adjunctive therapy, co-administered with either a selective serotonin reuptake inhibitor (SSRI) or a selective-norepinephrine reuptake inhibitor (SNRI), in subjects with major depressive disorder.

Inclusion Criteria:

1. Patients who are either inpatients or outpatients
2. Patients who have the ability to understand and to provide informed consent to the examination, observation, and evaluation processes specified in this protocol, and have signed the informed consent form based on a full understanding of the trial
3. Patients diagnosed as having either "296.2x Major Depressive Disorder, Single Episode" or "296.3x Major Depressive Disorder, Recurrent" according to DSM-IV-TR, and for whom the current episode of major depressive disorder has been ongoing for more than 8 weeks.

Exclusion Criteria:

1. Female patients of child bearing potential who wish to become pregnant during the treatment period, or within 4 weeks after study completion/discontinuation
2. Female patients who are pregnant, possibly pregnant, or breast feeding
3. Patients judged to be unable to tolerate any type of antidepressant treatment (including drugs not being used in the current episode of major depressive disorder), based on treatment history to date
4. Patients who have previously received electro-convulsive therapy
5. Patients who have participated in clinical studies on medical devices or other drugs within the past month
6. Patients at risk of having serious adverse events or developing symptoms that could interfere with safety and efficacy evaluations (such as symptoms of fibromyalgia syndrome overlapping with symptoms of depression), based on previous medical history
7. Patients with a history or a complication of diabetes
8. Patients with thyroid disease (excluding patients who are stabilized on drug therapy for at least 3 months)
9. Patients with a history of serotonin syndrome or psychotropic neuroleptic malignant syndrome
10. Patients with a history of seizure disorder (epilepsy etc.)