This is an 8-week, multicenter, randomized, double blind, placebo controlled study to evaluate the efficacy and safety of aripiprazole as an adjunctive treatment with mood stabilizer for the treatment of patients (outpatients or inpatients) with type I or II bipolar disorder accompanied by major depressive episode, without any psychotropic features.

This study involves patients who are considered by the investigator not to have a proper improvement, despite receiving a mood stabilizer (lithium or valproic acid) for a sufficient (≥ 28 days) period of time during the current depressive episode.

Inclusion Criteria:

• Patients aged ≥ 19 and < 70 years at the time of informed consent
• Patients who are able to understand information required for providing a consent
• Patients who have received a mood stabilizer (lithium or valproic acid)
• Patients with bipolar I or II disorder accompanied by major depressive episode
• Montgomery-Åsberg Depression Rating Scale (MADRS) total score ≥ 20 at both the screening and baseline visits

Exclusion Criteria:

• Patients diagnosed with bipolar I or II disorder with mania, mixed or psychotropic features
• Patients considered to have a high risk of suicide during the study period by the investigator based on current psychotic symptom and the patient's past medical history