The composition for optical materials includes a polymer obtained from silsesquioxanes which are represented by average composition formula (1): (R1SiO1.5)x(R2SiO1.5)y (wherein R1 is a polymerizable group, R2 is a non-polymerizable group, x is a number of 2.0 to 14.0, y is a number of 2.0 to 14.0, provided that x+y=8.0 to 16.0, and R1 groups and R2 groups may be the same or different) and include at least one cage silsesquioxane compound. This composition is suitable for use as the antireflective film in optical devices, has less film shrinkage in the curing step, has good coated surface state and excellent moisture resistance and adhesion, has small changes in the refractive index under high temperature conditions, and is capable of forming a low-refractive-index film.