A heat diffusion sheet 2 has a configuration in which a composite adhesive film is formed on a surface of a graphite sheet 10 having a thickness of 300 μm or more and 2,000 μm or less. This composite adhesive film has a configuration in which (A) an acrylic adhesive layer that has a thickness of 5 μm or more and 15 μm or less and does not contain a thermally conductive material, (B) a polyester film having a thickness of 20 μm or more and 60 μm or less, and (C) a silicone adhesive layer that has a thickness of 2 μm or more and 25 μm or less, does not contain a thermally conductive material, and has a peel strength of 0.005 N/cm or more and 1.0 N/cm or less are sequentially layered on the graphite sheet 10.