A semiconductor device substrate including: a substrate; a buffer layer which is provided on the substrate and made of a nitride semiconductor; and a device active layer which is formed of a nitride semiconductor layer provided on the buffer layer, the semiconductor device substrate in that the buffer layer includes: a first region which contains carbon and iron; a second region which is provided on the first region and has average concentration of iron lower than that in the first region and average concentration of carbon higher than that in the first region, and the average concentration of the carbon in the second region is lower than the average concentration of the iron in the first region. The semiconductor device substrate which can suppress a transverse leak current in a high-temperature operation of a device while suppressing a longitudinal leak current and can inhibit a current collapse phenomenon is provided.