A resist underlayer film-forming composition that exhibits a high etching resistance, a favorable dry etching rate ratio and optical constant, and can form a film exhibiting a good coatability even to a so-called uneven substrate, providing a small difference in film thickness after embedding, and having planarity and a superior hardness; a resist underlayer film formed from the resist underlayer film-forming composition; and a method of producing a semiconductor device. The composition including a reaction product between a compound of the following Formula (1) or (2) and a compound of the following Formula (3), and a solvent: