To provide a production method of a resin composition which does not foam even at a high temperature and has a low linear expansion coefficient and a high transparency; a resin composition prepared thereby; and an optical element using the composition.

The method produces a resin composition containing core-shell structured inorganic oxide particles in a resin and at least comprises a core particle dispersion process and a silica layer forming process. While maintaining the volume particle dispersion diameter of core particles in a range of 1-100 nm, the silica layer forming process clads the surface of the core particles with a silica layer in a liquid to prepare core-shell structured inorganic oxide particles (hereunder called as core-shell particles), and then the inorganic oxide particles and a resin satisfying the following formula are mixed. The formula is |Np-Nm|0.005, wherein Np is the flexibility of the core-shell particles, and Nm is the flexibility of the resin.