An object of the present invention is to provide a polyhedral polysiloxane composition that has high heat resistance and high light resistance, is excellent in gas-barrier properties and thermal shock resistance, and exhibits good handleability when used to encapsulate an optical semiconductor device. The polyhedral polysiloxane composition of the present invention is characterized by including a modified polyhedral polysiloxane which is obtained by hydrosilylation of an alkenyl group-containing polyhedral polysiloxane compound (a) and a hydrosilyl group-containing compound (b) and has a structure derived from an organic silicon compound (a′) having one alkenyl group per molecule.