A semiconductor device includes: a first semiconductor layer having an N conductive type and made of a gallium oxide-based semiconductor; and a second semiconductor layer made of a gallium oxide-based semiconductor, in contact with the first semiconductor layer, and having the N conductive type with an electrically active donor concentration higher than an electrically active donor concentration of the first semiconductor layer. A difference between a donor concentration of the first semiconductor layer and a donor concentration of the second semiconductor layer is smaller than a difference between the electrically active donor concentration of the first semiconductor layer and the electrically active donor concentration of the second semiconductor layer.