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 Alzheimer's 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 Alzheimer's disease, comprising the step of administering the antibody； and a method for suppressing progression of Alzheimer's disease, comprising the step of administering the antibody.
