To provide an aqueous ink binder having good printability as an aqueous ink for wall paper and giving a resulting coated surface being excellent in weathering resistance, water resistance and scratching resistance.

An aqueous printing ink binder for wall paper comprises an aqueous dispersion of a copolymer (A) which is formed by the emulsion polymerization of a monomer mixture comprising 0.5-10 mass % of a monomer (a1) containing a carboxy group and 99.5-90 mass % of the other copolymerizable monomer (a2) in the presence of an emulsifying agent (a3) and a polymerization initiator (a4) in an aqueous medium, and has an average particle diameter of 0.1-0.5 μm, a glass transition temperature of -10 to 40°C and a solubility parameter of 9.8-10.4.