The present invention relates to [1] a process for producing pigment-containing modified polymer particles, including the step of reacting pigment-containing polymer particles (A) containing a functional group and a compound (B) containing a reactive group capable of reacting with the functional group of the polymer particles (A) in a medium under such a condition that a ratio [(B)/(A)] of total moles of the reactive group of the compound (B) to total moles of the functional group of the polymer particles (A) is from 0.10 to 0.62; [2] a pigment water dispersion including an aqueous medium and the modified polymer particles produced by the aforementioned process which are dispersed in the aqueous medium; and [3] an ink including the aforementioned pigment water dispersion and an organic solvent. The modified polymer particles are free from formation of coarse particles upon production of pigment particles, so that an ink obtained by using the modified polymer particles can be prevented from suffering from increase in viscosity thereof when the ink is being concentrated by evaporation of water from the ink, and is excellent in rub fastness when printed on a low-water absorbing recording medium.