To provide a viscosity improver capable of giving excellent finishability(smoothness, vividness, sagging-proofness).

The viscosity improver comprises a copolymer of the following monomers(a) to (d) as the essential constituents. The monomer(a) is (meth)acrylic acid or a salt thereof. The monomer(b) is represented by the formula(1)( wherein, R1is H or methyl; R2and R3are each H, methyl or ethyl; R4is a 8-24C alkyl; and n is 3-60 ). The monomer(c) is a 1-4C alkyl (meth)acrylate. The monomer(d) is a polyfunctional one. In this copolymer, the (a) unit content, the (b) unit content, the (c) unit content and the (d) unit content are 0.1-50 wt.%, 5-60 wt.%, 5-60 wt.% and 0.05-5 wt.%, based on the total weight of the monomer units(a) to (d), respectively.

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