This invention relates to the use of a copolymer of polymerizable unsaturated monomer containing at least one kind of ionic functional group selected from the group consisting of quaternary ammonium salt group and sulfonic acid group, nonionic polymerizable unsaturated monomer having polyoxyalkylene chain, and other ethylenically unsaturated monomer, as a pigment dispersing resin for water-based paint or water-based ink.