A toner is disclosed, for developing electrostatic images by means of image forming apparatuses, that comprises toner particles, and an external additive, wherein the toner particles comprise a binder resin and a colorant, the external additive is introduced onto the surface of the toner particles, the external additive liberates from the surface of the toner particles in a rate of 7% to 50% under the condition that the toner is dispersed within a surfactant-containing electrolyte at 20 W output power and 20 kHz frequency for one minute by means of an ultrasonic homogenizer.