There is provided a resist underlayer film forming composition for forming a resist underlayer film providing heat resistance properties and hardmask characteristics. A resist underlayer film forming composition for lithography, comprising: a polymer containing a unit structure of Formula (1): O—Ar1  Formula (1) (in Formula (1), Ar1 is a C6-50 arylene group or an organic group containing a heterocyclic group), a unit structure of Formula (2): O—Ar2—O—Ar3-T-Ar4  Formula (2) (in Formula (2), Ar2, Ar3, and Ar4 are individually a C6-50 arylene group or an organic group containing a heterocyclic group; and T is a carbonyl group or a sulfonyl group), or a combination of the unit structure of Formula (1) and the unit structure of Formula (2). The organic groups of Ar1 and Ar2 containing arylene group may be organic groups containing a fluorene structure.