An object of the invention is to provide a peptide based on a sequence of an amyloid β peptide that may allow for induction of enhanced immune response and is safe and efficacious for prophylaxis and treatment of Alzheimer disease. An amyloid β peptide or a portion thereof with addition or insertion of cysteine or a cysteine analogue, and a method for enhancing immune response to amyloid β using said peptide, a medicament for prophylaxis and treatment of Alzheimer disease using said amyloid β peptide that induces an enhanced immune response, and a DNA vaccine comprising a gene coding for an amyloid β peptide or a sequence derived from an amyloid β peptide with addition or insertion of cysteine or a cysteine analogue, as expected to be similarly efficacious.