To provide a viscosity-improving agent imparting an excellent finishing property (smoothness and clear shining property), drip-preventing property and solvent resistance.

This viscosity-improving agent is characterized by containing (A) a urethane compound expressed by general formula (1) and (C) a polyoxyalkylene compound expressed by general formula (2) [wherein, R is a hydrocarbon group; Y is a reaction residue obtained by eliminating isocyanate group from diisocyanate; OA, AO are each the oxyalkylene group; X is H or a hydrocarbon group; (m), (n) are each 20-500; (f) is 200-25,000; and in each of the compounds, at least 80 wt.% of the total weight of the oxyalkylene groups is oxyethylene group]. Based on the weight of the (A) and (C), it is preferable to have 10-90 wt.% content of each of them.

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