In a semiconductor device including a core transistor and an I/O transistor on the same semiconductor substrate, the core transistor includes a gate insulating film, a gate electrode, sidewalls, extension diffusion layers, and source/drain diffusion layers. The I/O transistor includes a gate insulating film, a gate electrode, sidewalls, and source/drain diffusion layers. In the I/O transistor, the source/drain diffusion region is offset relative to a channel region located beneath the gate insulating film in regions below the sidewalls.