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 Alzheimer's 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 Alzheimer's disease, which comprises a step of administering the antibody; and a method for preventing the progression of Alzheimer's disease, which comprises a step of administering the antibody.