A platelet coagulation inhibitor which inhibits neither COX-1 nor COX-2 is provided. The inhibitor is a compound represented by general formula (I):

wherein Ar 1 and Ar 2 independently represent a 5- or 6-membered aromatic heterocyclic group optionally substituted with 1 to 3 substituents, or a phenyl group optionally substituted with 1 to 3 substituents; R1 represents a lower acyl group, carboxyl group, a lower alkoxycarbonyl group, a lower alkoxy group, a lower alkyl group optionally substituted with 1 or 2 substituents, a carbamoyl group optionally substituted with 1 or 2 substituents, an oxamoyl group optionally substituted with 1 or 2 substituents, an amino group optionally substituted with 1 or 2 substituents, a 4- to 7-membered alicyclic heterocyclic group optionally substituted with 1 or 2 substituents, a phenyl group optionally substituted with 1 to 3 substituents, or a 5- or 6-membered aromatic heterocyclic group optionally substituted with 1 to 3 substituents; and R2 represents hydrogen atom, a halogeno group, or the like.