To provide a viscosity-improving agent, capable of imparting excellent finishing properties (smoothness, clarity, sag-preventing property, etc.).

This viscosity improving agent is provided by containing a copolymer of (a) a (meth)acrylic acid (salt), (b) a monomer expressed by formula (1) [wherein, R1 is H or methyl; R2, R3 are each H, methyl or ethyl; R4 is an aromatic hydrocarbon group; and (n) is an integer of 3 to 60] and (c) an alkyl (meth)acrylate in which the number of carbon atoms of the alkyl group is 1 to 4, as essential constituting monomers and having 0.1 to 50 wt.% content of the unit (a), 5 to 60 wt.% content of the unit (b) and 5 to 60 wt.% content of the unit (c) based on the total weight of each of the monomer units (a) to (c).