A therapeutic agent suitable for treating cognitive impairment induced by amyloid β protein, comprising as an effective component a peptide corresponding to any one of the following to: a peptide consisting of the amino acid sequence represented by SEQ ID NO:1: VLSSQQFLHRGHQPPPEMAGHSLASSHRNSMIPSAAT (SEQ ID NO:1); a peptide consisting of the same amino acid sequence as the amino acid sequence represented by SEQ ID NO:I except that one to three amino acids are altered by any one or more of deletion, addition, substitution, and side-chain modification; a peptide consisting of a partial sequence of the amino acid sequence represented by SEQ ID NO:I, comprising the amino acid sequence represented by SEQ ID NO:2: HRGHQPPPEMA (SEQ ID NO:2); a peptide consisting of the same amino acid sequence as a partial sequence of the amino acid sequence represented by SEQ ID NO:1 comprising the amino acid sequence represented by SEQ ID NO:2, except that one or two amino acids are altered by any one or more of deletion, addition, substitution, and side-chain modification; a peptide consisting of an amino acid sequence comprising the amino acid sequence represented by SEQ ID NO:2, wherein a total of 1 to 50 amino acids are added to the N-terminus and/or the C-terminus of the amino acid sequence represented by SEQ ID NO:2 (excluding amino acid sequences which are identical to all or part of the amino acid sequence represented by SEQ ID NO:1); and a peptide consisting of an amino acid sequence comprising the same amino acid sequence as the amino acid sequence represented by SEQ ID NO: 2 except that one or two amino acids are altered by any one or more of deletion, addition (excluding addition to the N-terminus and/or the C-terminus), substitution, and side-chain modification, wherein a total of 1 to 50 amino acids are added to the N-terminus and/or the C-terminus of said altered amino acid sequence, for use in therapy. a peptide consisting of the amino acid sequence represented by SEQ ID NO:1: VLSSQQFLHRGHQPPPEMAGHSLASSHRNSMIPSAAT (SEQ ID NO:1); a peptide consisting of the same amino acid sequence as the amino acid sequence represented by SEQ ID NO:I except that one to three amino acids are altered by any one or more of deletion, addition, substitution, and side-chain modification; a peptide consisting of a partial sequence of the amino acid sequence represented by SEQ ID NO:I, comprising the amino acid sequence represented by SEQ ID NO:2: HRGHQPPPEMA (SEQ ID NO:2); a peptide consisting of the same amino acid sequence as a partial sequence of the amino acid sequence represented by SEQ ID NO:1 comprising the amino acid sequence represented by SEQ ID NO:2, except that one or two amino acids are altered by any one or more of deletion, addition, substitution, and side-chain modification; a peptide consisting of an amino acid sequence comprising the amino acid sequence represented by SEQ ID NO:2, wherein a total of 1 to 50 amino acids are added to the N-terminus and/or the C-terminus of the amino acid sequence represented by SEQ ID NO:2 (excluding amino acid sequences which are identical to all or part of the amino acid sequence represented by SEQ ID NO:1); and a peptide consisting of an amino acid sequence comprising the same amino acid sequence as the amino acid sequence represented by SEQ ID NO: 2 except that one or two amino acids are altered by any one or more of deletion, addition (excluding addition to the N-terminus and/or the C-terminus), substitution, and side-chain modification, wherein a total of 1 to 50 amino acids are added to the N-terminus and/or the C-terminus of said altered amino acid sequence, for use in therapy.