To provide a viscosity index-improving agent having excellent viscosity index-improving ability for an ester-based synthetic lubricant.

The viscosity index-improving agent for ester-based synthetic lubricants is composed of a copolymer (A) containing (a1) one or more kinds of (meth)acrylates selected from a group consisting of a (meth)acrylic acid alkyl ester having a 1-4C alkyl group and a (meth)acrylic acid alkyl ester having 1-4C hydroxyalkyl group, (a2) a (meth)acrylic acid alkyl ester having an 11-15C alkyl group and (a3) a (meth)acrylic acid alkyl ester having a 16-24C alkyl group as essential constituent monomers, and solubility parameter of the copolymer (A) is 9.3-11.