An object of the present invention is to provide a platelet aggregation inhibitor without side effects and a supplement food effective for inhibiting platelet aggregation. The platelet aggregation inhibitor has nattokinase as an active ingredient and has Bacillus natto culture extract, containing a high proportion of nattokinase containing 1 g/g or less of vitamin K₂ on a dry weight basis, as an active ingredient. The supplement food effective for inhibiting platelet aggregation has nattokinase as an active ingredient and has Bacillus natto culture extract, containing a high proportion of nattokinase containing 1 g/g or less of vitamin K₂ on a dry weight basis, as an active ingredient.