A composition for forming a resist underlayer film to be used in a lithography process, that includes: a polymer containing unit structures of Formula (1), Formula (2), and Formula (3): the polymer being a polymer in which the unit structure of Formula (1) has a ratio of mole number (a) within a range of 0.20≦a≦0.90, the unit structure of Formula (2) has a ratio of mole number (b) within a range of 0.05≦b≦0.60, and the unit structure of Formula (3) has a ratio of mole number (c) within a range of 0.001≦c≦0.40, when a total mole number of all unit structures constituting the polymer is 1.0, and the polymer having a weight average molecular weight of 3,000 to 100,000; a crosslinkable compound; a photoacid generator; and a solvent.