1. A semiconductor integrated circuit device comprising: a high-speed storage unit which includes a first storage device, and a second storage device having a type different from that of said first storage device; a control unit which controls said first storage device and said second storage device; and a power supply unit which supplies source voltage independently to said first storage device and said second storage device, said first storage device being operable at a voltage below the lower limit voltage at which said second storage device is operable, said control unit operating switchable controls between a first mode allowing only said first storage device of said high-speed storage unit to operate, and a second mode allowing said first storage device or said second storage device of said high-speed storage unit to operate, and said power supply unit, in said first mode, operating to supply a voltage below the lower limit voltage at which said second storage device of said high-speed storage unit is operable, to said first storage device of said high-speed storage unit, while interrupting power supply to said second storage device of said high-speed storage unit.