To provide a viscosity index-improving agent which has a more improved viscosity index-improving effect than those of conventional agents.

This viscosity index-improving agent comprises a vinylic polymer (A) containing a vinylic monomer (a) represented by the general formula (1) [R1 is H or a 1 to 32C alkyl; R2 is a 1 to 32C alkyl; A is a 2 to 4C alkylene; (n) is an integer of 1 to 200] as an essential constituting monomer; a viscosity index-improving agent composition which comprises the viscosity index-improving agent, and a diluent and/or other additives, and is useful for lubricants; and a lubricant composition containing the viscosity index-improving agent in an amount of 0.03 to 30 wt% based on the weight of the lubricant composition.