A resist underlayer film-forming composition for lithography process having characteristics of enabling wafer surface planarization after film formation, excellent planarization performance on substrate with level difference, and good embeddability in fine hole pattern. The resist underlayer film-forming composition including polymer having unit structure of Formula (1) and solvent, wherein each of R1 to R4 is independently hydrogen atom or methyl group, and X1 is divalent organic group having at least one arylene group optionally substituted by alkyl group, amino group, or hydroxyl group, and wherein X1 in Formula (1) is organic group of Formula (2), wherein A1 is phenylene group or naphthylene group, A2 is phenylene group, naphthylene group, or organic group of Formula (3), and dotted line is bond, and wherein each of A3 and A4 is independently phenylene group or naphthylene group, and dotted line is bond.