In a semiconductor device, an active region includes: a first impurity region to which a predetermined voltage is applied; second and third impurity regions forming a pair of conductive electrodes of an insulated gate field effect transistor; and at least one impurity region disposed between the first and second impurity regions. A voltage that causes electrical conduction between the second and third impurity regions is applied to a gate electrode disposed between the second and third impurity regions. All gate electrodes disposed between the first and second impurity regions are configured to be electrically connected to the first impurity region constantly. All impurity regions disposed between the first and second impurity regions are electrically isolated from the first and second impurity regions and maintained in a floating state.