A heterocyclic compound according to claim 3, wherein each of R1 and R2 is a methyl group; R3 is a hydrogen atom; each of R4 and R5 which may be the same or different, is a hydrogen atom or a methyl group; each of Y's is a hydrogen or halogen atom, or an alkyl, haloalkyl or haloalkoxyl group; R6 is an alkyl, haloalkyl or cycloalkyl group, or ##STR168## (in which each of V's is a hydrogen or haloge