The present invention relates to a thrombin receptor antagonist containing a compound represented by the formula (I) wherein R1a, R1b and R2 are each a hydrogen atom, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group, or an optionally substituted alkoxy, R3 is a group represented by the formula —NHCOR4, —NHSO2R5, —NHCON(R6a)(R6b), —NHCOOR7 or —CONHR8 wherein R4, R5, R6a, R6b, R7 and R8 are each a hydrogen atom, an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group and the like), ring A is monocyclic aromatic ring optionally further having substituent(s), R1a and R1b are optionally bonded to each other to form an optionally substituted nitrogen-containing non-aromatic heterocycle, or a salt thereof or a prodrug thereof. The thrombin receptor antagonist of the present invention has a thrombin receptor (particularly PAR-1) antagonistic action and is useful for the prophylaxis or treatment of PAR-1 mediated pathological condition or disease.