To obtain a viscosity index improver having at least excellent shear stability and readily improving the viscosity index as compared with conventional viscosity improvers.

The viscosity index improver comprises a polymer obtained by polymerizing an alkyl (meth)acrylate having a 12-24C alkyl group and having 10,000-35,000 weight-average molecular weight as a base polymer. The viscosity index improver is contained in a lubricating oil comprising a base oil and the viscosity index improver so as to provide (1/4) to (1/1) polymer/base oil (weight ratio). The base oil is preferably a high-viscosity index oil having 110-150 viscosity index. A polymerization initiator used when synthesizing the polymer is more preferably hydrophobic. Furthermore, the lubricating oil comprising the viscosity index improver and the base oil is provided.