Abstract: A method for manufacturing a semiconductor substrate, including: applying a composition for forming a resist underlayer film directly or indirectly to a substrate to form a resist underlayer film directly or indirectly on the substrate; forming a resist pattern directly or indirectly on the resist underlayer film; and performing etching using the resist pattern as a mask. The composition for forming a resist underlayer film contains: a polymer having a repeating unit represented by formula (1) and a solvent. Ar1 is a divalent group including an aromatic ring having 5 to 40 ring atoms; and R0 is a monovalent group including an aromatic ring having 5 to 40 ring atoms and includes at least one group selected from the group consisting of groups represented by formula (2-1) and groups represented by formula (2-2).