There is provided a high voltage semiconductor device comprising: a semiconductor substrate of a first conductivity type, including a first region, a second region relatively lower than the first region, and a sloped region between the first region and the second region; a drift region of a second conductivity type, formed on the second region; a source region of the second conductivity type, disposed on the first region, and spaced apart from the drift region by the sloped region; a drain region of the second conductivity type, disposed on the drift region; a field plate positioned on the drift region in the second region; a gate insulating layer disposed between the source region and the drift region; and a gate electrode layer, which is disposed on the gate insulating layer and extends to above the field plate.