An anti-cognitive dysfunction agent comprising a humanized antibody which does not bind to A² monomers and specifically binds only to A² oligomers and a fragment thereof as an active ingredient, and an therapeutic antibody which can be treat Alzheimer's disease by specifically binding amyloid ² protein oligomer (A² oligomer) which is considered to be a cause of Alzheimer's disease are required. The present invention can provide an anti-A² 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 A² 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.
