There is provided a resist underlayer film-forming composition having excellent solubility in a solvent containing propylene glycol monomethyl ether as a main component. A resist underlayer film-forming composition comprising a polymer having a structural unit of Formula (1a) or Formula (1c) and a structural unit of Formula (1b) and a solvent containing more than 50% by mass of propylene glycol monomethyl ether, wherein in the polymer, the structural unit of Formula (1a) or Formula (1c) and the structural unit of Formula (1b) are arranged alternately. (In Formulae (1a) and (1b), Q is a phenylene group or a naphthylene group, m is 1 or 2, and each of n is independently 0 or 1).