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, and to a pharmaceutical composition and an amyloid β fibrillogenesis inhibitor comprising the peptide.
