A viscosity index improver including a comb-shaped polymer is provided. For a solution having the viscosity index improver dissolved in a mineral oil and having a solid component concentration of 25 mass %, a ratio of the storage modulus (G′) to the loss modulus (G″) of the solution measured at a measuring temperature of 70° C. is 0.40 or more. For a solution (α) at 25° C. having the viscosity index improver dissolved in a mineral oil and having a solid component concentration of 25 mass % and a solution (β) resulting from subjecting the solution (α) to heating to 100° C. at a prescribed temperature rise rate and then cooling to 25° C. at a prescribed cooling rate, a ratio of the storage modulus (G′) of the solution (β) to the storage modulus (G′) of the solution (α) measured at a measuring temperature of 25° C. is 2.0 or more.