A substrate for semiconductor device includes a substrate, a buffer layer which is provided on the substrate and made of a nitride semiconductor, and a device active layer which is provided on the buffer layer and composed of a nitride semiconductor layer, wherein the buffer layer contains carbon and iron, a carbon concentration of an upper surface of the buffer layer is higher than a carbon concentration of a lower surface of the buffer layer, and an iron concentration of the upper surface of the buffer layer is lower than an iron concentration of the lower surface of the buffer layer. As a result, the substrate for semiconductor device can reduce a leak current in a lateral direction at the time of a high-temperature operation while suppressing a leak current in a longitudinal direction.