Toner for developing electrostatic image and image formation process using it

A toner for developing an electrostatic image comprises a binding resin for a toner and a charge control agent including an azo-type iron complex salt represented by the following chemical formula (1) (in the chemical formula (1), R1 and R3 are the same or different to each other and are an alkyl group having a straight chain or a branch chain of 3 to 8 carbons; R2a, R2b, R4a and R4b are the same or different to each other and are selected from the group consisting of a hydrogen atom, an alkyl group, an alkoxyl group, a halogen atom, a nitro group and a carboxyl group; A+ indicates m(H+)+n(K+)+p(Na+), which m, n and p satisfy numerical equations of m+n+p=1, 0.7 m 1, 0 n 0.3 and 0 p 0.3), wherein an average particle size thereof is 1 to 4 microns, and a specific volume resistivity thereof is ranging from 0.2×1015 to 7×1015 ·cm.