A screening method directed towards； (1) specifying a receptor binding site by introduction of mutation, and (2) specifying a binding site-specific peptide motif on the basis of an amino acid selection ratio by contrast between a peptide motif bound to a wild-type subunit and a peptide motif bound to a mutant functionally deficient in the binding site according to a peptide library method. A peptide which inhibits a toxin whose receptor binding portion has a subunit structure is screened. Accordingly, an STX2 inhibitor in which an STX2 inhibiting peptide is incorporated in a molecular nuclear structure portion having three molecules of lysine (Lys) peptide-linked thereto and which is easy to synthesize and can inhibit verotoxin is provided.
