To provide a siloxane-based resin composition capable of formimg a cured film high in refractive index and in visible light transmission, and excellent in crack resistance after a thermal load.

The resin composition contains a condensation product and a thermally crosslinkable compound represented by general formula (1) or (3), wherein, the former is prepared by acid catalyzed hydrolysis of an alkoxysilane compound in the presence of particles of a compound of at least one sort of metal selected from aluminium, tin, titanium, and zirconium in a solvent, and by successive condensation reaction.