A platelet aggregation inducing substance containing as an active ingredient a polypeptide having a peptide fragment represented by formula (1) (component A) : €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ-(Pro-X-Gly) n -€ƒ€ƒ€ƒ€ƒ€ƒ(1) wherein X represents Pro or Hyp; and n represents an integer of from 20 to 5,000.