Encapsulating sheet and optical semiconductor element device

An optical semiconductor element device comprising:

• an optical semiconductor element and
• an encapsulating layer which is formed from an encapsulating sheet and encapsulates the optical semiconductor element, and
• the encapsulating sheet comprising:
• an encapsulating resin composition which contains an encapsulating resin including an encapsulating silicone resin composition, and silicone microparticles, wherein
• the mixing ratio of the silicone microparticles with respect to the encapsulating resin composition is 20 to 50 mass %, and
• the silicone microparticles are microparticles of polysiloxanes having a cross-linked structure,
• the encapsulating resin is a transparent resin,
• the refractive index of the silicone microparticles is 1.39 to 1.43, and
• a difference between the refractive index of the silicone microparticles and the refractive index of the encapsulating resin is within 0.03 in absolute value.