A composite semiconductor substrate being able to improve voltage withstanding and crystalline quality is provided. A composite semiconductor substrate is equipped with an Si (silicon) substrate, an SiC (silicon carbide) layer formed on the surface of the Si substrate, an AlN (aluminum nitride) layer formed on the surface of the SiC layer, a composite layer formed on the surface of the AlN layer, and a GaN (gallium nitride) layer formed on the surface of the composite layer. The composite layer includes an AlN (aluminum nitride) layer and a GaN layer formed on the surface of the AlN layer. In at least one composite layer, the average density of C and Fe in the GaN layer is higher than the average density of C and Fe in the AlN layer.