The present invention relates to a novel antiplatelet agent and a novel compound which is an active ingredient for the agent. The present invention provides the antiplatelet agent comprising a compound represented by the formula I: wherein, X is N, or CR1d, Xb1-Xb5 are the same or different, and are nitrogen or carbon, R1a-R1d are the same or different, and are hydrogen, an optionally substituted alkyl, an optionally substituted alkoxy, an optionally substituted alkylthio, an alkenyl, a cycloalkyl, a halogen, cyano, or hydroxyl or optionally substituted by 1 or 2 alkylamino, R2 is an optionally substituted aryl or an optionally substituted heteroaryl, R3 is an optionally substituted aryl or an optionally substituted heteroaryl, or pharmaceutically acceptable salt thereof as an active ingredient.