A heterocyclic compound useful as an antiallergic agent is provided.A compound represented by the following formula (1) or a salt thereof: wherein the ring A is a homocyclic or heterocyclic ring; the ring B is a heterocyclic ring which contains G and nitrogen atom N as constituent atoms thereof, wherein G is CH or N; R1 is a carbonyl group or an alkylene group; R2a and R2b are an alkyl group, a cycloalkyl group, an aryl group, or a heterocyclic group; X is an oxygen atom or a sulfur atom; Z is a hydroxyl group, an alkoxy group, a cycloalkyloxy group, an aryloxy group, an aralkyloxy group, an amino group, or an N-substituted amino group; and n is 0 or 1; with the proviso that when the ring A is a benzene ring or when the ring B is a piperazine ring, R1 is an alkylene group which may have a substituent.