The present invention provides a peptide which functions as a mimic peptide of an amyloid β peptide and is capable of inhibiting the fibrillogenesis of an amyloid β peptide. The present invention relates to: an 8- to 30-amino acid residue peptide comprising an amino acid sequence represented by the following formula (I):X1-Asp-X2-X3-X4-Pro-X5-X6 (SEQ ID NO: 28) (I)wherein X1 represents a branched chain amino acid, and X2, X3, X4, X5, and X6 are the same or different and each represents an α-amino acid a 9- to 30-amino acid residue peptide comprising an amino acid sequence represented by the following formula (III):Z1-X13-Gly-X14-X15-Pro-Trp-Met-Z2 (SEQ ID NO: 31) (III)wherein X13, X14, and X15 are the same or different and each represents an α-amino acid, Z1 and Z2 are the same or different and each represents cysteine or serine, and when Z1 and Z2 represent cysteine, they may be crosslinked and a pharmaceutical composition and an amyloid β fibrillogenesis inhibitor comprising the peptide.
