According to one embodiment, a semiconductor device includes a semiconductor substrate made of a first semiconductor material, an element isolation insulating film, a gate electrode film, source/drain regions, a channel region, and a diffusion preventing film. The channel region is provided near a surface of the semiconductor substrate below the gate electrode film, and containing a second impurity of a predetermined conductivity type diffused therein. The diffusion preventing film is provided at an interface between the element isolation insulating film and the semiconductor substrate, and made of a second semiconductor material different from the first semiconductor material.