MOS integrated circuit device operating with low power consumption

In accordance with selection signals corresponding to an operation mode from a mode detection circuit, the voltage levels of back gate voltages applied to the back gates of MOS transistors included in internal circuitry are selected, by the selection signals, among the voltages from voltage generation circuits for generating a plurality of voltages having different voltage levels. The threshold voltage and the drive current of the MOS transistor are adjusted in accordance with the operation mode, and the semiconductor integrated circuit device which operates at high speed with low current consumption can be achieved.