A MOS transistor including a source region, a drain region, and a gate electrode has first and second partial isolation regions in one-end gate region and the other-end gate region, respectively, with a first tap region provided adjacent to the first partial isolation region, and a second tap region provided adjacent to the second partial isolation region. A full isolation region is provided in the whole area around the first and second partial isolation regions, first and second tap regions, and source and drain regions.