Fused heterocyclic compound and thrombopoietin receptor activator

Disclosed are fused heterocyclic compounds that are useful for prophylaxis, therapy or amelioration of diseases on which a thrombopoietin receptor activation effect is effective. Specifically disclosed are a fused heterocyclic compound represented by formula (I) (wherein R 1 represents an aryl group that is fused to a saturated ring, or the like; and A, B, L 1 , R 2 , L 2 , L 3 , Y, L 4 , R 3 and X are as defined in the description), a tautomer, prodrug or pharmaceutically acceptable salt of the compound, and a solvate of the compound, tautomer, prodrug or pharmaceutically acceptable salt.