A viscosity index improver composition, comprising: a copolymer containing a polyolefin-based monomer (a) represented by the following formula, a monomer (b) represented by the following formula and a (meth)acrylic acid alkyl ester (e) having a C1-C4 alkyl group as constituent monomers; a copolymer containing a (meth)acrylic acid alkyl ester (c) having a C12-C15 straight-chain or branched alkyl group and a (meth)acrylic acid alkyl ester (d) having a C16-C20 straight-chain or branched alkyl group as constituent monomers; and a base oil, wherein the weight average molecular weight ratio { /} of the copolymer to the copolymer is 2 to 55, the weight ratio (A/B) of the copolymer to the copolymer constituting the viscosity index improver composition is 5 to 100, the copolymer is a copolymer containing, as constituent monomers, the monomer (a) in an amount of 5 to 40 wt%, the monomer (b) in an amount of 3 to 60 wt%, and the (meth)acrylic acid alkyl ester (e) in an amount of 30 to 85 wt% based on the weight of the copolymer, the copolymer has a weight average molecular weight of 20,000 to 100,000, and the copolymer is a copolymer containing, as constituent monomers, the (meth)acrylic acid alkyl ester (c) in an amount of 50 to 98 wt% and the (meth)acrylic acid alkyl ester (d) in an amount of 2 to 50 wt% based on the weight of the copolymer: wherein R 1 is a hydrogen atom or a methyl group; -X 1 - is a group represented by -O-, -O m -, or -NH-, A is a C2-C4 alkylene group, m is an integer of 1 to 10, and each A may be the same or different when m is 2 or greater; R 2 is a residue after removal of one hydrogen atom from a hydrocarbon polymer containing at least one of an isobutylene group or a 1,2-butylene group as a structural unit; and p represents a number of 0 or 1, wherein R 3 is a hydrogen atom or a methyl group; -X 2 - is a group represented by -O- or -NH-; R 4 is a C2-C4 alkylene group; R 5 is a C1-C8 alkyl group; and q is an integer of 1 to 20, and each R 4 may be the same or different when q is 2 or greater. a copolymer containing a polyolefin-based monomer (a) represented by the following formula, a monomer (b) represented by the following formula and a (meth)acrylic acid alkyl ester (e) having a C1-C4 alkyl group as constituent monomers; a copolymer containing a (meth)acrylic acid alkyl ester (c) having a C12-C15 straight-chain or branched alkyl group and a (meth)acrylic acid alkyl ester (d) having a C16-C20 straight-chain or branched alkyl group as constituent monomers; and a base oil, wherein the weight average molecular weight ratio { /} of the copolymer to the copolymer is 2 to 55, the weight ratio (A/B) of the copolymer to the copolymer constituting the viscosity index improver composition is 5 to 100, the copolymer is a copolymer containing, as constituent monomers, the monomer (a) in an amount of 5 to 40 wt%, the monomer (b) in an amount of 3 to 60 wt%, and the (meth)acrylic acid alkyl ester (e) in an amount of 30 to 85 wt% based on the weight of the copolymer, the copolymer has a weight average molecular weight of 20,000 to 100,000, and the copolymer is a copolymer containing, as constituent monomers, the (meth)acrylic acid alkyl ester (c) in an amount of 50 to 98 wt% and the (meth)acrylic acid alkyl ester (d) in an amount of 2 to 50 wt% based on the weight of the copolymer: wherein R 1 is a hydrogen atom or a methyl group; -X 1 - is a group represented by -O-, -O m -, or -NH-, A is a C2-C4 alkylene group, m is an integer of 1 to 10, and each A may be the same or different when m is 2 or greater; R 2 is a residue after removal of one hydrogen atom from a hydrocarbon polymer containing at least one of an isobutylene group or a 1,2-butylene group as a structural unit; and p represents a number of 0 or 1, wherein R 3 is a hydrogen atom or a methyl group; -X 2 - is a group represented by -O- or -NH-; R 4 is a C2-C4 alkylene group; R 5 is a C1-C8 alkyl group; and q is an integer of 1 to 20, and each R 4 may be the same or different when q is 2 or greater.