A resist underlayer film forming composition contains a resin containing a unit structure represented by formula (1): [in formula (1), R1 represents a thiadiazole group which is optionally substituted with a C1-6 alkyl group optionally interrupted by a carboxy group, a C1-6 alkyl group optionally substituted with a hydroxyl group, or a C1-4 alkylthio group, and R2 represents a hydrogen atom or formula (2): (in formula (2), R1 is the same as defined above, and * represents a binding moiety)]. The resist underlayer film forming composition provides a resist underlayer film which has excellent solvent resistance, excellent optical parameters, an excellent dry etching rate, and excellent embeddability.