A humanized antibody that cannot bind to an Aβ monomer but can bind only to an Aβ oligomer specifically and an antibody drug that comprises the antibody as an active ingredient, can bind specifically to an amyloid-β protein oligomer (an Aβ oligomer) (which is believed to be a causative protein of cognitive impairment or Alzheimers disease) and therefore can treat Alzheimers disease have been demanded. Disclosed are: a humanized anti-Aβ oligomer antibody which can bind to an Aβ oligomer specifically and a method for treating Alzheimers disease utilizing the humanized antibody. Specifically disclosed are: a therapeutic agent for Alzheimers disease, a senile plaque formation inhibitor or an Aβ-amyloid fibril formation inhibitor 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.Se describe un anticuerpo humanizado anti-oligómero de Aß que no se une a los monómeros de Aß y que solamente se une específicamente a los oligómeros de Aß un agente anti-disfunción cognoscitiva, un agente para tratar la enfermedad de Alzheimer, un agente para suprimir la formación de placa neurítica y un inhibidor de la formación de fibra de amiloide Aß que comprende el anticuerpo como ingrediente activo un método para por lo menos uno de prevención y tratamiento de la disfunción cognoscitiva o enfermedad de Alzheimer, que comprende la etapa de administrar el anticuerpo y un método para suprimir la progresión de la enfermedad de Alzheimer, que comprende la etapa de administrar el anticuerpo.
