PURPOSE: To prevent increase of leak current at the time of reversed bias of a gate to a source or a drain when a junction type FET is irradiated with radiation.

CONSTITUTION: In a junction type FET which has a source region 4 of a second conductivity type and a drain region 5 of a second conductivity type in a semiconductor layer 3 of a first conductivity type, and has a thermal oxide film 9 and a nitride film 10 as insulating films, high concentration regions 11 and 12 of a second conductivity type are arranged in the source region 4 and the drain region 5, respectively. When positive holes as positive charges are generated in the thermal oxide film by the irradiation of radiation, the high concentration regions 11 and 12 in the source region 4 and the drain region 5 are not inverted, so that increase of leak current at the time of backward bias is restrained.