Providing a semiconductor device with SOI structure having a substrate doping profile by which the threshold voltage can be set at a low level for realizing low-voltage operations of a MOSFET under operating conditions and at a high level for reducing off leakage current in the off-state. A semiconductor device with SOI structure comprises a SOI substrate, source/drain regions 6 and 7 formed, and a gate insulating film 24 and a gate electrode 11. The thickness of the source/drain regions 6 and 7 is smaller than that of the surface semiconductor layer 3. The channel region has a first conductivity type high-density impurity diffused layer 5. |