It is intended to provide a platelet aggregation inhibitor which can be easily processed into a pharmaceutical preparation and can be stored for a long time, and a health food effective in inhibiting platelet aggregation. The platelet aggregation inhibitor comprises, as an active ingredient, a degradation product obtained by reacting fibrin with nattokinase, wherein the degradation product has a molecular weight of 10,000 daltons or less. The fibrin is obtained by reacting fibrinogen with thrombin.