There is provided a resist underlayer film forming composition for lithography for forming a resist underlayer film capable of being used as a hard mask or a bottom anti-reflective coating, or a resist underlayer film causing no intermixing with a resist and having a dry etching rate higher than that of the resist. A film forming composition comprising a silane compound having an onium group, wherein the silane compound having an onium group is a hydrolyzable organosilane having, in a molecule thereof, an onium group, a hydrolysis product thereof, or a hydrolysis-condensation product thereof. The composition uses as a resist underlayer film forming composition for lithography. A composition comprising a silane compound having an onium group, and a silane compound having no onium group, wherein the silane compound having an onium group exists in the whole silane compound at a ratio of less than 1% by mol, for example 0.01 to 0.95% by mol. The hydrolyzable organosilane may be a compound of Formula: R1aR2bSi(R3)4−(a+b). A resist underlayer film obtained by applying the composition as claimed in any one of claims 1 to 14 onto a semiconductor substrate and by baking the composition.