Viscosity index improver composition and lubricating oil composition

A viscosity index improver composition, comprising a comb-shaped polymer (A) comprising a structural unit derived from a macromonomer (α) and a structural unit derived from a monomer (A1) represented by the following general formula (a1):

• wherein in the general formula (a1):
• R11 represents a hydrogen atom or a methyl group;
• R12 represents a single bond, —O—, or —NH—;
• R13 represents a linear alkylene group having 2 to 4 carbon atoms or a branched alkylene group having 2 to 4 carbon atoms;
• m1 represents an integer of 0 to 20, in which in the case where m1 represents an integer of 2 or more, plural groups represented by R13 may be the same as or different from each other, and the (R13O)m1 moiety may be either random copolymerization or block copolymerization;
• and
• R14 represents an alkyl group having 4 to 8 carbon atoms except for a n-butyl group, and
• wherein the macromonomer (α) has a (meth)acryloyl group at one end, and has a structural unit derived from one or more kinds of a monomer (α1) selected from butadiene and hydrogenated butadiene.