A package base for a semiconductor element is made of a carbon composite material. The mounting surface of the package base includes a first area on which at least one first element including at least one semiconductor element is to be mounted, and a second area on which at least one second element including at least a terminal for electrode wiring is to be mounted. Preferably, the carbon composite material is a high-density, isotropic carbon-fiber composite material. In addition, the mounting surface may have a step change in surface elevation for alignment of either of the at least one first element and the at least one second element.