t which comprises employing as said toner (d) at least one member selected from the group consisting of compounds represented by the formula ##STR4## wherein R1 is a hydrogen atom, a C1 -C5 straight chain or branched alkyl group, an unsubstituted or substituted phenyl group or an unsubstituted or substituted C3 -C8 cycloalkyl group; R2 is a C1 -C5 straight chain or branched alkyl group, an unsubst