A resist underlayer film-forming composition having a dramatically improved crosslinking ability over conventional compositions, and further, a resist underlayer film-forming composition that crosslinks with a component of a resist material, in order to improve the adhesion of a resist underlayer film to a resist pattern. A resist underlayer film-forming composition for lithography including a copolymer having a structural unit of formula (1) and a structural unit of formula (2):




(wherein each R1 is independently a hydrogen atom or a methyl group; each R2 is independently a C1-3 alkylene group; R3 is a single bond or a methylene group; A is a linear, branched, or cyclic aliphatic group having a carbon atom number of 1 to 12 and optionally having a substituent, or a C6-16 aromatic or heterocyclic group optionally having a substituent; and Pr is a protecting group); a crosslinking agent; an organic acid catalyst; and a solvent.