To provide a thermosetting composition giving a cured product excellent in heat resistance and lightfastness, and to provide an optical semiconductor device formed by sealing an optical semiconductor element by using the composition.

The thermosetting composition comprises an aluminosiloxane or aluminosiloxane having a photopolymerizable functional group having an ethylenical double bond, and an epoxy silicone with an epoxy equivalent of 500-5,000 g/mol. The optical semiconductor device constructed by sealing the optical semiconductor element by using the thermosetting composition is also provided.