A semiconductor device includes a semiconductor substrate formed of at least two kinds of group III elements and nitrogen, an active layer formed on the semiconductor substrate, and a nitride semiconductor layer formed on a surface of the semiconductor substrate and formed between the semiconductor substrate and the active layer. The nitride semiconductor layer is formed of the same constituent elements of the semiconductor substrate. A composition ratio of the lightest element among the group III elements of the nitride semiconductor layer is higher than a composition ratio of the corresponding element of the semiconductor substrate.