Abstract: A semiconductor device comprises a composite substrate comprising a semiconductor substrate and a semiconductor layer on said semiconductor substrate with a dielectric layer interposed therebetween; a plurality of element regions formed in the semiconductor layer and each having formed a field effect transistor including a source region and a drain region of a first conduction type; and an impurity-diffused region of a second conduction type which is formed directly under an element isolating film isolating respective elements. The impurity-diffused region having the opposite conduction type and formed under the element separating film restrain formation of parasitic transistors and prevent a decrease in threshold value.