The present invention provides a lubricating oil additive, including: a molybdenum compound (A) represented by the following general formula (1); and a sulfur-based composition (B) containing a sulfur-based compound (b-1) represented by the following general formula (2) and at least one kind of sulfur-based compound (b-2) represented by the following general formula (3) or (4), and a lubricating oil composition containing the additive. In the formula (1), R1 to R4 each represent alkyl groups having 6 to 18 carbon atoms, the groups being identical to or different from each other, and X1 to X4 each independently represent an oxygen atom or a sulfur atom. In the formula (2), R5 represents an alkyl group having 1 to 22 carbon atoms, R6 represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, R7 represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and “a” represents a number from 1 to 10. In the formulae (3) and (4), R8 to R11 each represent alkylene groups having 1 to 28 carbon atoms, the groups being identical to or different from each other, R12 and R13 each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and “b” and “c” each independently represent a number from 1 to 8, provided that at least one of R12 or R13 represents an alkyl group having 1 to 3 carbon atoms.