A resist underlayer film-forming composition containing a copolymer having a structural unit of Formula (1) and a structural unit of Formula (2), a cross-linkable compound, a cross-linking catalyst, and a solvent. wherein A is a divalent organic group containing a triazine ring, X1 is an —S— group or an —O— group, Q is a linear, branched, or cyclic hydrocarbon group having a carbon atom number of 1 to 15, the hydrocarbon group may have at least one sulfur atom or oxygen atom in a main chain and at least one hydroxy group as a substituent, n is 0 or 1, R1 and R2 are each independently a C1-3 alkylene group or a single bond, Z is a divalent group having at least one sulfur atom or oxygen atom, and when X1 is an —O— group, Z is a divalent group having at least one sulfur atom.