To provide a pigment dispersing agent for aqueous inks which gives an aqueous ink composition excellent in printing density and the like while maintaining the same pigment dispersibility as the dispersibility of the conventionally known pigment dispersing agent for aqueous inks.

The pigment dispersing agent for aqueous inks includes a polymer obtained by polymerizing an unsaturated carboxylic acid and/or an unsaturated sulfonic acid, a styrene and/or a (meth)acrylic ester of a hydrophobic monomer, and optionally a copolymerizable monomer at a specific ratio, and the polymer has at least one of the structures: "-S-C(=S)-", "-S-C(=S)-S-", and "-SH", a number average molecular weight in terms of polystyrene by gel permeation chromatography of 2,000-40,000, a ratio of the weight average molecular weight to the number average molecular weight of 1.8 or less, and a sum of the ratio of the number average molecular weight of less than 500 and the ratio of not less than 200,000 of less than 5% based on the total number of molecules.