A toner for developing an electrostatic image of the present invention contains toner particles obtained by forming coagulated particles by mixing a resin particle dispersion solution containing dispersed resin particles and a coloring agent dispersion solution containing dispersed coloring agent particles and fusing the coagulated particles by heating them and is characterized in that at least the surfaces of the toner particles have a chemical structure formed by reaction with a compound having a carbodiimido group. The invention also provides a resin particle dispersion solution to be used for the production of toner.