There is provided an underlayer coating that is used as an underlayer of photoresists in lithography process of the manufacture of semiconductor devices and that has a high dry etching rate in comparison to the photoresists depending on the type of etching gas, does not intermix with the photoresists, and is capable of flattening the surface of a semiconductor substrate having holes of a high aspect ratio; and an underlayer coating forming composition for forming the underlayer coating. The underlayer coating forming composition for forming by light irradiation an underlayer coating used as an underlayer of a photoresist in a lithography process of the manufacture of semiconductor devices, includes a polymerizable compound containing 5 to 45% by mass of silicon atom (A), a photopolymerization initiator (B), and a solvent (C).