A carbon fiber composite sheet obtained by combining a carbon fiber aggregate with a thermosetting resin component selected from the group consisting of thermosetting silicone-based gel components, thermosetting epoxy resin components and thermosetting silicone resin components, and then curing the thermosetting resin component, the carbon fiber composite sheet having crystallite sizes of at least 5 nm in the direction of growth of the hexagonal mesh surface of the carbon fibers composing the carbon fiber aggregate, and the carbon fiber composite sheet having a thermal conductivity of at least 2 W/(m·K).