The present invention relates to a viscometric properties improver capable of improving the fuel efficiency of an internal combustion engine, and an additive composition for a liquid coolant containing the viscometric properties improver. [1] A viscometric properties improver consisting of a compound (A) represented by the following formula (1) and a compound (B) represented by the following formula (2), and [2] an additive composition for a liquid coolant containing the same: R1OR2OmSO3M  (1) R3O—SO3M  (2) wherein R1 and R3 are the same as or different from each other and each represent a linear or branched alkyl group or alkenyl group having 12 or more and 24 or less carbon atoms; R2 represents an ethylene group or a propylene group; m represents an addition molar number of R2O of 1 or more and 15 or less; and M represents a cation or a hydrogen atom.