The present invention is a nitride semiconductor device including an n-type gallium nitride single crystal substrate, an epitaxially grown nitride film on the substrate, and electrodes deposited on a top and a bottom of the substrate. In order to produce the substrate, oxygen is doped into a gallium nitride crystal by preparing a C-plane gallium nitride seed crystal or a three-rotationally symmetric plane foreign material seed crystal, supplying material gases including gallium, nitrogen and oxygen to the C-plane gallium nitride seed crystal or the three-rotationally symmetric foreign seed crystal, growing a faceted C-plane gallium nitride bulk crystal having facets of non-C-planes on the seed crystal, maintaining the facets on the C-plane gallium nitride bulk crystal, and eliminating the seed crystal from the bulk crystal.