Provided is an antibody or an immunologically reactive fragment thereof which targets exclusively an Aβ having a specific turn structure of Aβ (a toxic conformer of Aβ). Also provided are a medicinal composition comprising, as an active ingredient, an antibody that specifically recognizes a toxic conformer of Aβ, a kit for assaying a toxic conformer of Aβ, a diagnostic agent for Alzheimer's disease, etc.
