The present invention provides an optical refractive index-modifying polymer composition which can efficiently modulate (change) refractive index upon light irradiation, and exhibits such high storage stability that the once modulated refractive index does not substantially change anymore upon time elapse, and the optical refractive index-modifying polymer composition comprises as a main component a polymer (A) which is a polymer of monomers including an acrylic vinyl monomer represented by the following formula (1): CH2═C(R1)—C(═O)O—R2═CH2   (1) wherein R1 represents a hydrogen atom or a methyl group, R2 represents a saturated or unsaturated hydrocarbon group having 1 to 20 carbon atoms, and the molecule may contain a hetero atom or a halogen atom, as an essential component and contains a remaining radical-polymerizable side-chain vinyl group in the molecule, wherein the composition comprises a thermally curable polymer (B) in an amount of 5 to 60 parts by weight per 100 parts by weight of the polymer (A).