There is provided a resist underlayer film having heat resistance that is used for a lithography process in the production of semiconductor devices, and a high refractive index film having transparency that is used for an electronic device. A polymer comprising a unit structure of Formula (1): wherein each of R1, R2, R3, and R5 may be a hydrogen atom, R4 may be phenyl group or naphthyl group. A resist underlayer film forming composition comprising the polymer, and a resist underlayer film formed from the composition. A high refractive index film forming composition comprising the polymer, and a high refractive index film formed from the composition.