An anti-Aβ oligomer humanized antibody which does not bind to Aβ monomers and specifically binds only to Aβ oligomers an anti-cognitive dysfunction agent, an agent for treating Alzheimers disease, an agent for suppressing formation of neuritic plaque and an inhibitor of formation of Aβ amyloid fiber comprising the antibody as an active ingredient a method for at least one of preventing and treating cognitive dysfunction or Alzheimers disease, comprising the step of administering the antibody and a method for suppressing progression of Alzheimers disease, comprising the step of administering the antibody.
