To provide a viscosity index improver good in compatibility with a GTL(gas-to-liquids) base oil and excellent in viscosity index-improving ability.

The viscosity index improver for GTL base oil comprises (A) a copolymer 5,000-1,000,000 in weight-average molecular weight, wherein a monomer(a) of the general formula(1) is the essential constituent monomer. The general formula(1) is CH2=C(R1)-COO(A-O)n-(CH2)p-CH(R2)-R3( wherein, R1 is H or methyl; R2 and R3 are each a 1-16C straight-chain alkyl or 3-34C branched alkyl, wherein R2 and R3 may be the same as or different from each other, the number of the consecutive methylene groups in the branched alkyl group is 16 or less, and the total number of the carbon atoms in R2, R3 and p CH2 groups is 18-36; A is a 2-4C alkylene group; n is an integer of 1-20; and p is an integer of 1-15 ).