To provide a viscosity index improver having excellent shear stability, ability to improve the viscosity index and low-temperature viscosity.

The viscosity index improver is composed of a copolymer (A) comprising an alkyl (meth)acrylate (a) as an essential constituent monomer. The alkyl (meth)acrylate (a) is represented by general formula (1) {wherein, R1 is a hydrogen atom or a methyl group; the alkyl group moiety in formula (2) has 28-80 carbon number; R2 and R3 are each a hydrogen atom or a 1-3C alkyl group; R4 and R5 are each a hydrogen atom or a 1-3C alkyl group, with the proviso that R3 and R5 are not simultaneously a hydrogen atom; n and k are each an integer of 4-36; A1 is a 2-4C alkylene group; and m is 0 or an integer of 1-20}.