A semiconductor device is, constituted by: a nitride group semiconductor functional layer which includes a first nitride group semiconductor region, a second nitride group semiconductor region provided on the first nitride group semiconductor region by a hetero junction, and a two-dimensional carrier gas channel near the hetero junction of the first nitride group semiconductor region; a first main electrode and a second main electrode connected to the two-dimensional carrier gas channel by ohmic contact; and a gate electrode disposed between the first main electrode and the second main electrode. The nitride group semiconductor region has different thicknesses between the second main electrode and the gate electrode, and between the first main electrode and the gate electrode.