PROBLEM TO BE SOLVED: To provide a new platelet aggregation inhibitor which can reduce or prevent various symptoms caused by the aggregation of platelets, and particularly, to provide a platelet aggregation inhibitor which can be more easily obtained and whose mass production can be easily achieved as well.SOLUTION: The platelet aggregation inhibitor includes at least one selected from 2-decenoic acid, 4-decenoic acid and 9-decenoic acid as an active constituent. Further, a blood flow improving agent or an antithrombotic agent using the platelet aggregation inhibitor as an active constituent is provided.