A resin composition of the present invention is used for forming a filling material which fills at least a through-hole of a semiconductor substrate, the through-hole extending through the semiconductor substrate in a thickness direction thereof and having a conductive portion therein. The resin composition is composed of: a resin having a radical-polymerizable double bond, a thermosetting resin and a resin which differs from the resin having the radical-polymerizable double bond and has an alkali-soluble group and a double bond; or a cyclic olefin resin. A filling material of the present invention is formed of a cured product of the above resin composition. An insulating layer of the present invention is formed of a cured product of the above resin composition. The insulating layer includes: a layer-shaped insulating portion provided on a surface opposite to a functional surface of the semiconductor substrate; and a filling portion integrally formed with the insulating portion and filling the through-hole. A semiconductor device of the present invention includes the above insulating layer.