An anti-A² oligomer humanized antibody which does not bind to A² monomers and specifically binds only to A² 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 A² 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.
