To provide a viscosity index improving agent good in compatibility with a GTL (gas to liquids) base oil and superior in viscosity index improving capability.

The viscosity index improving agent for GTL base oil comprises a copolymer (A) having a monomer (a) expressed by general formula (1), as an essential constituent monomer, and having a weight-average molecular weight of 5,000-1,000,000. In the formula, R1 is a hydrogen atom or a methyl group; R2 and R3 are each a 1-16C straight chain alkyl group or a 3-34C branched alkyl group; R2 and R3 may be mutually the same or different; the number of continuing methylene groups in the branched alkyl group is 16 or less; the total number of carbons of R2, R3 and p pieces of CH2 groups is 18-36; A is a 2-4C alkylene group; n is 0 or an integer of 1-20 and p is 0 or an integer of 1-15.