The purpose of the present invention is to provide a thiodicarboxylic acid ester which has high long-term storage stability without inhibiting antioxidant properties and wear-resistant properties of the thiodicarboxylic acid ester.In order to achieve the above-mentioned purpose, the present invention provides a lubricating oil additive composition which is characterized by containing a compound (A) represented by general formula (1) and a compound (B) represented by general formula (2) and by having an acid value of 0.01-0.4 mgKOH/g. (in the formula, R1 and R4 each independently represent a hydrocarbon group having 6 to 18 carbon atoms and R2 and R3 each independently represent an alkylene group having 1 to 4 carbon atoms) (in the formula, R5 represents a hydrocarbon group having 6 to 18 carbon atoms and R6 and R7 each independently represent an alkylene group having 1 to 4 carbon atoms).