A Study Versus E2020 10mg Followed by an Open-label Extension Phase to Explore the Safety of E2020 SR 23 mg in Japanese Subjects With Severe Alzheimer's Type Dementia

Inclusion Criteria * Diagnostic evidence of probable Alzheimer's disease (AD) consistent with the Diagnostic and Statistical Manual for Mental Disorders-version IV (DSM-IV) * Hachinski Ischemic Score * Functional Assessment Staging (FAST) scale greater than or equal to 6 at Screening. * Mini-Mental State Examination (MMSE) score of 1 to 12 at Screening * Subjects who are on a stable Aricept- dose of 10 mg immediate release (IR), taken as a single, daily dose for 3 months prior to the Screening Visit * Evidence consistent with Alzheimer's disease (AD) on any cranial image on magnetic resonance imaging (MRI) or computed tomography (CT) scan or etc. obtained within 24 months prior to the Screening Visit. Subjects who have any observations of dementia other than Alzheimer's type after the last image diagnosis should be reconfirmed. * Age 50 years * Written informed consent is to have been obtained from the subject (if possible) or from the subject's legal guardian or other representative Exclusion Criteria * Subjects with dementia other than Alzheimer's type * Subjects with significant neurological or psychiatric disorders such as stroke, brain tumor, schizophrenia, epilepsy, normal pressure hydrocephalus, mental retardation, a history of head injury with loss of consciousness, or a history of brain surgery followed by persistent deficits * Subjects with allergy to donepezil hydrochloride or piperidine derivatives * Subjects with a cause of Alzheimer's disease (AD) which is supported by any laboratory tests such as Vitamin B12, folate levels, triiodothyronine, free triiodothyronine, thyroxine, thyroid stimulating hormone (TSH) or serologic test for syphilis