When a crystal layer of III-V Group nitride compound semiconductor is formed, a nitride compound semiconductor layer is first overlaid on a substrate to form a base layer and a III-V Group nitride compound semiconductor represented by the general formula In_xGa_yAl_zN (where 0x1, 0y1, 0z1, x+y+z=1) is epitaxially grown on the base layer by hydride vapor phase epitaxy at a deposition pressure of not lower than 800 Torr. By making the deposition pressure not lower than 800 Torr, the crystallinity of the III-V Group nitride compound semiconductor can be markedly improved and its defect density reduced.