An object of the present invention is to provide a semiconductor device and a manufacturing method thereof which can realize a normally-off field-effect transistor made of a III group nitride semiconductor. The present invention includes: placing a sapphire substrate in a crystal growth chamber; forming a low-temperature GaN buffer layer made of GaN as the III group nitride semiconductor, on a main surface of the sapphire substrate by a MOCVD method; and forming a GaN layer on the low-temperature GaN buffer layer by the MOCVD method. Here, a [11-20] axis of the GaN layer is perpendicular to the main surface of the sapphire substrate.