A Multi-Center, Randomized, Double-Blind, Parallel Group Study With 3 Groups Receiving Placebo, 5 mg of E2020 and 10 mg of E2020

Exclusion Criteria

• •Patients with dementia other than AD.
• •1. Patient whose imaging test shows "circumscribed cerebral lesion or multiple infarcts" which is suspected to be the responsible cause of dementia (mixed-type dementia)
• •2. Patients with other types of dementia
• •Patients with a current DSM-IV diagnosis of major depressive disorder or any current serious psychiatric diagnosis othe than AD.
• •Patients without a reliable caregiver.
• •Patients who are expected to enter a nursing home facility within 6 months after starting study treatment (excluding a transient entry).
• •Patients with active or clinically significant conditions affecting absorption, distribution or metabolism of study drugs (e.g., inflammatory bowel disease, gastric or duodenal ulcers, or severe lactose intolerance).
• •Patients with a known hypersensitivity to a component of donepezil hydrochloride preparation or piperidine derivatives.
• •Patients complicated with a severe gastrointestinal, hepatic, renal, endocrine, or cardiovascular system disease (e.g., sick sinus syndrome, intraarterial and supraventricular conduction disorder, etc.).
• •Patients with a history of severe bronchial asthma or obstructive pulmonary disease.
• •Patients with a severe extrapyramidal disease (Parkinson's disease, Parkinson's syndrome, etc.).
• •Patients with diabetes mellitus with uncontrolled blood glucose levels (HbAlc of ≧10%).
• •Hypertension patients with uncontrolled blood pressure (diastolic blood pressure of ≧95 mmHg).
• •Patients with uncontrolled thyroid dysfunction.
• •Patients with a history of seizure or convulsion within the previous 3 months (obtaining informed consent).
• •Patients with a known or suspected history of alcohoism or drug dependence within the recent 10 years.
• •Patients with malignant tumors.
• •Women of pregnant, possibly pregnant, or lactating.
• •Patients who have participated in another clinical study within the recent 3 months (before giving consent).
• •Any other patients who are judged to be ineligible for study entry by the investigator or subinvestigator.