Disclosed is a method of treatment for anti-Alzheimer's disease based on an action mechanism associated with amyloid β protein, which action mechanism is different from conventional action mechanisms. The treatment Alzheimer's disease uses a therapeutic agent for cognitive impairment induced by amyloid β protein, which therapeutic agent comprises a peptide having the amino acid sequence represented by SEQ ID NO:1 or a peptide similar to this peptide, especially a peptide containing the amino acid sequence represented by SEQ ID NO:2, which is a partial sequence of SEQ ID NO:1. VLSSQQFLHRGHQPPPEMAGHSLASSHRNSMIPSAAT (SEQ ID NO:1) HRGHQPPPEMA (SEQ ID NO:2).