A semiconductor device includes a substrate, a semiconductor layer provided on the substrate and having a plurality of GaN channel layers and a plurality of AlGaN barrier layers which are alternately laminated from a substrate side, a source electrode and a drain electrode electrically connected to the GaN channel layers, and a gate electrode provided between the source electrode and the drain electrode to control a potential of the semiconductor layer, wherein an Al composition ratio of an AlGaN barrier layer closest to the substrate is smaller than that of an AlGaN barrier layer second closest to the substrate.