According to an embodiment, a semiconductor device includes a gate electrode formed on a semiconductor substrate via an insulating layer; a source region including an extension region, a drain region including an extension region, a first diffusion restraining layer configured to prevent a diffusion of the conductive impurity in the source region and including an impurity other than the conductive impurity, and a second diffusion restraining layer configured to prevent a diffusion of the impurity in the drain region and including the impurity other than the conductive impurity.