A humanized antibody which cannot bind to an Aβ monomer but can bind only to an Aβ oligomer specifically an anti-cognitive impairment agent, a therapeutic agent for Alzheimers disease, a senile plaque formation inhibitor or an Aβ-amyloid fibril formation inhibitor, each of which comprises the antibody as an active ingredient a method for preventing and/or treating cognitive impairment or Alzheimers disease, which comprises a step of administering the antibody and a method for preventing the progression of Alzheimers disease, which comprises a step of administering the antibody.
