Sealing agent for optical semiconductor device, and optical semiconductor device

The present invention provides a sealing agent for optical semiconductor devices exhibiting excellent curability and storage stability, as well as high heat resistance in a cured state. The sealing agent for optical semiconductor devices according to the present invention includes: a first organopolysiloxane represented by formula (1A) and having an aryl group and an alkenyl group; a second organopolysiloxane represented by formula (51) and having an aryl group and a hydrogen atom bonded to a silicon atom; and a catalyst for a hydrosilylation reaction. The content ratio of aryl groups in each of the first organopolysiloxane and second organopolysiloxane is 30 mol%-70 mol% inclusive.