In one embodiment of the present invention, a high withstand voltage transistor is disclosed having small sizes including an element isolating region. The semiconductor device is provided with the element isolating region formed on a semiconductor substrate; an active region demarcated by the element isolating region; a gate electrode formed on the semiconductor substrate in the active region by having a gate insulating film in between; a channel region arranged in the semiconductor substrate under the gate electrode; a source region and a drain region positioned on the both sides of the gate electrode; and a drift region positioned between one of or both of the source region and the drain region and the channel region. One of or both of the source region and the drain region are at least partially positioned on the element isolating region, and are connected with the channel region through the drift region.