To provide a composition for forming a resist underlayer film used in lithography, the composition forming a resist underlayer film capable of being used as a hard mask. A composition for forming a resist underlayer used in lithography containing, as a silane, a hydrolyzable silane, a hydrolyzate of said silane, or a hydrolysis condensate of said silane. The hydrolyzable silane contains a silane of formula (1) or a combination of formula (1) and formula (2). The silane of formula (1) or the combination of formula (1) and formula (2) constitutes less than 50 mol% on the basis of the total silane content. In formula (1), R1 is an organic group containing formula (1-1), formula (1-2), formula (1-3), formula (1-4), or formula (1-5); a indicates an integer of 1, b indicates an integer of 0 or 1; a+b indicates an integer of 1 or 2. In formula (2), R4 is an organic group containing formula (2-1), formula (2-2), or formula (2-3); a 1 indicates an integer of 1; b 1 indicates an integer of 0 or 1, and a1+b1 indicates an integer of 1 or 2.