The present invention provides Ti-4 protein, which is a protein obtained from salivary gland of Triatoma infestans, and the present invention provides Ti-4 gene encoding the protein. As the Ti-4 protein exhibits inhibitory activity on platelet aggregation, a medicine comprising the Ti-4 protein as an active ingredient will serve as a platelet aggregation inhibitor possibly effective for prevention and treatment of myocardial infarction, pulmonary infarction and cerebral infarction. Moreover, the Ti-4 protein will also serve as a highly prospective lead compound in the development of novel platelet aggregation inhibitors.