A lubricating oil additive composition comprising a poly(meth)acrylate-based copolymer (X) containing a structural unit (a) derived from an alkyl (meth)acrylate (A) represented by the following general formula (a-1), a structural unit (b) derived from a hydroxy group-containing (meth)acrylate (B) represented by the following general formula (b-1), and a structural unit (c) derived from a phosphorus-containing (meth)acrylate (C) represented by the following general formula (c-1): wherein in the general formula (a-1), R a1 represents a hydrogen atom or a methyl group; and R a2 represents an alkyl group having 8 to 20 carbon atoms, wherein in the general formula (b-1), R b1 represents a hydrogen atom or a methyl group; R b2 represents an alkylene group having 2 to 4 carbon atoms; and m1 represents an integer of 1 to 10, in which in the case where m1 represents an integer of 2 or more, plural groups represented by R b2 may be the same as or different from each other, wherein in the general formula (c-1), R c1 represents a hydrogen atom or a methyl group; R c2 represents an ethylene group; m2 represents an integer of 1 to 6, in which in the case where m2 represents an integer of 2 or more, plural groups represented by R c2 may be the same as or different from each other; n represents an integer of 1 or 2; and in the case where n = 1, at least one of plural groups represented by R c3 is a hydrogen atom and the other one is selected from any of a hydrogen atom, a methyl group or an ethyl group, and in the case where n = 2, R c3 represents a hydrogen atom, wherein the total content of the structural units (a), (b), and (c) of the poly(meth)acrylate-based copolymer (X) is 70% by mass to 100% by mass, based on the total structural units of the poly(meth)acrylate-based copolymer (X), and wherein the poly(meth)acrylate-based copolymer (X) satisfies the following condition (α): <Condition (α)> a content ratio [(a)/(b)] of the structural unit (a) and the structural unit (b) in terms of molar ratio of 20/80 to 80/20. a structural unit (a) derived from an alkyl (meth)acrylate (A) represented by the following general formula (a-1), a structural unit (b) derived from a hydroxy group-containing (meth)acrylate (B) represented by the following general formula (b-1), and a structural unit (c) derived from a phosphorus-containing (meth)acrylate (C) represented by the following general formula (c-1): wherein in the general formula (a-1), R a1 represents a hydrogen atom or a methyl group; and R a2 represents an alkyl group having 8 to 20 carbon atoms, wherein in the general formula (b-1), R b1 represents a hydrogen atom or a methyl group; R b2 represents an alkylene group having 2 to 4 carbon atoms; and m1 represents an integer of 1 to 10, in which in the case where m1 represents an integer of 2 or more, plural groups represented by R b2 may be the same as or different from each other, wherein in the general formula (c-1), R c1 represents a hydrogen atom or a methyl group; R c2 represents an ethylene group; m2 represents an integer of 1 to 6, in which in the case where m2 represents an integer of 2 or more, plural groups represented by R c2 may be the same as or different from each other; n represents an integer of 1 or 2; and in the case where n = 1, at least one of plural groups represented by R c3 is a hydrogen atom and the other one is selected from any of a hydrogen atom, a methyl group or an ethyl group, and in the case where n = 2, R c3 represents a hydrogen atom, wherein the total content of the structural units (a), (b), and (c) of the poly(meth)acrylate-based copolymer (X) is 70% by mass to 100% by mass, based on the total structural units of the poly(meth)acrylate-based copolymer (X), and wherein the poly(meth)acrylate-based copolymer (X) satisfies the following condition (α): <Condition (α)> a content ratio [(a)/(b)] of the structural unit (a) and the structural unit (b) in terms of molar ratio of 20/80 to 80/20. <EOS>