Resist underlayer film forming composition

A resist underlayer film forming composition comprising a polymer having a repeating structural unit of formula (1a) and/or (1b):

• [wherein two R1s are each independently a C1-10 alkyl group, a C2-6 alkenyl group, an aromatic hydrocarbon group, a halogen atom, a nitro group, or an amino group, two R2s are each independently a hydrogen atom, a C1-10 alkyl group, a C2-6 alkenyl group, an acetal group, an acyl group, or a glycidyl group, R3 is an aromatic hydrocarbon group optionally having a substituent or a heterocyclic group, R4 is a hydrogen atom, a phenyl group, or a naphthyl group, when R3 and R4 bonded to the same carbon atom are each a phenyl group, R3 and R4 are optionally bonded to each other to form a fluorene ring, two ks are each independently 0 or 1, m is an integer of 3 to 500, p is an integer of 3 to 500, X is a benzene ring, and two —C(CH3)2— groups bonded to the benzene ring are in a meta position or a para position], and a solvent.