Provided is a therapeutic method exclusively targeting an amyloid β protein (Aβ) having a specific turn structure of Aβ. Specifically provided is an antibody which specifically recognizes an amyloid β having a turn structure at amino acids positions 22 and 23. Also provided are a medicinal composition comprising, as the active ingredient, an antibody specifically recognizing a toxic conformer of amyloid β, an assay kit for a toxic conformer of amyloid β, a diagnostic for Alzheimers disease, etc.
