An anti-cognitive dysfunction agent comprising a humanized antibody which does not bind to Aβ monomers and specifically binds only to Aβ oligomers and a fragment thereof as an active ingredient, and an therapeutic antibody which can be treat Alzheimer's disease by specifically binding amyloid β protein oligomer (Aβ oligomer) which is considered to be a cause of Alzheimer's disease are required. The present invention can provide an anti-Aβ oligomer humanized antibody and a method for treating Alzheimer's disease using the humanized antibody. An agent for treating Alzheimer's disease； an agent for suppressing formation of neuritic plaque； an inhibitor of formation of Aβ amyloid fiber； a method for at least one of preventing and treating cognitive dysfunction or Alzheimer's disease, comprising the step of administering the humanized antibody； and a method for suppressing progression of Alzheimer's disease, comprising the step of administering the humanized antibody.
