A region for substrate potential is formed of an n-type well at a position in the direction of a channel length relative to the gate electrode and the position is between drain regions in the direction of a channel width. An n-type of a contact region with a higher concentration of n-type impurity than that of the region is provided in the region. The contact region is arranged away from the drain regions with a distance to obtain a desired breakdown voltage of PN-junction between the region and the drain region.