To provide a composition for a silicone resin capable of providing the silicone resin showing thermoplastic and thermosetting properties, and also excellent in light fastness and heat resistance, a resin composition obtained by reacting the composition, a photosemiconductor element-sealing material containing the composition and the photosemiconductor device sealed by the sealing material.

This composition for a silicone resin containing a component associated with the 2 kinds of reactions of the reaction of an isocyanate group and hydrosililation reaction is provided by containing a both terminals amino type silicone resin, an organohydrogen siloxane, a diisocyanate and a hydrosililation catalyst.