In a semiconductor device having an active region and an inactive region, the active region includes a channel forming layer with a heterojunction structure having first and second semiconductor layers, a gate structure portion having a MOS gate electrode, a source electrode and a drain electrode disposed on the second semiconductor layer with the gate structure portion interposed therebetween, a third semiconductor layer disposed at a position away from the drain electrode between the gate structure portion and the drain electrode and not doped with an impurity, a p-type fourth semiconductor layer disposed on the third semiconductor layer, and a junction gate electrode brought into contact with the fourth semiconductor layer. The junction gate electrode is electrically connected to the source electrode to have a same potential as a potential of the source electrode, and is disposed only in the active region.