A compound semiconductor device is manufactured by using a polycrystalline SiC substrate, the compound semiconductor device having a buffer layer being formed on the substrate and having a high thermal conductivity of SiC and aligned orientations of crystal axes. The method for manufacturing the compound semiconductor device includes: forming a mask pattern on a polycrystalline SiC substrate, the mask pattern having an opening of a stripe shape defined by opposing parallel sides or a hexagonal shape having an apex angle of 120 degrees and exposing the surface of the polycrystalline SiC substrate in the opening; growing a nitride semiconductor buffer layer, starting growing on the polycrystalline SiC substrate exposed in the opening of the mask pattern, burying the mask pattern, and having a flat surface; and growing a GaN series compound semiconductor layer on the nitride semiconductor buffer layer.