A gallium nitride based semiconductor device is provided which includes a gallium nitride based semiconductor film with a flat c-plane surface provided on a gallium oxide wafer. A light emitting diode LED includes a gallium oxide support base 32 having a primary surface 32a of monoclinic gallium oxide, and a laminate structure 33 of Group III nitride. A semiconductor mesa of the laminate structure 33 includes a low-temperature GaN buffer layer 35, an n-type GaN layer 37, an active layer 39 of a quantum well structure, and a p-type gallium nitride based semiconductor layer 37. The p-type gallium nitride based semiconductor layer 37 includes, for example, a p-type AlGaN electron block layer and a p-type GaN contact layer. The primary surface 32a of the gallium oxide support base 32 is inclined at an angle of not less than 2 degrees and not more than 4 degrees relative to a (100) plane of monoclinic gallium oxide. Owing to this inclination, the gallium nitride based semiconductor epitaxially grown on the primary surface 32a of the gallium oxide support base has a flat surface.