A semiconductor device is provided with: a field-effect transistor that has a source electrode and a drain electrode that are connected to a semiconductor layer, a gate electrode that is provided on the surface of the semiconductor layer between the source electrode and the drain electrode, and a field plate electrode that is provided on the surface of the semiconductor layer in the vicinity of the gate electrode via an insulating layer, wherein the field-effect transistor amplifies high frequency signals received by the gate electrode to be outputted from the drain electrode; and a voltage dividing circuit that divides a potential difference between the drain electrode and a reference potential GND, and applies a bias voltage such that respective parts of the field plate electrode have a mutually equal potential.