An additive for a resist underlayer film-forming composition that modifies a surface state of a resist underlayer film into a hydrophobic state to enhance adhesion between the resist underlayer film and a resist pattern formed on the resist underlayer film, and a resist underlayer film-forming composition containing the additive. An additive for a resist underlayer film-forming composition including a polymer having a structural unit of Formula (1):
(wherein R1 is a hydrogen atom or methyl group, L is a single bond or a divalent linking group, and X is an aliphatic hydrocarbon group, an alicyclic hydrocarbon group, or a heterocyclic group having an oxygen atom as a heteroatom that does not have a hydroxy group). A resist underlayer film-forming composition for lithography including a resin binder, an organic solvent, and the additive.