To provide a semiconductor device which obtains rapid operation and reduction in a static power supply current while restraining an increase in circuit scale and an increase in element area.

A semiconductor device has an MOSFET circuit 2 provided on an SOI substrate, a power supply circuit part 1, which supplies a power supply voltage V2 to the MOSFET circuit 2 and a power supply voltage control circuit 3, which supplies a control signal Scnt1 to the power supply circuit part 1 and controls the power supply voltage V2. Thereby, it is possible to obtain rapid operation by setting the power supply voltage V2 at a high voltage during the operation of the MOSFET circuit 2 and to reduce a static power supply current by setting the power supply voltage V2 at a low voltage during the operation stop of the MOSFET circuit 2. Additionally, since a low voltage is also supplied in an operation stop, data, which is volatizably stored in the MOSFET circuit 2, can be maintained since the low voltage is supplied also during the operation stop.