In an integrated circuit device, third transistors having the thickest gate insulation film are driven at high voltage and thus operate at high speed with minimal gate leak current. First transistors having the thinnest gate insulation film and second transistors which do not have the thinnest gate insulation film are driven at low voltage, the second transistors being driven at all times and the first transistors being halted as appropriate. The second transistors operate constantly at low speed and with minimal gate leak current, and the first transistors, which have significant gate leak current, operate at high speed while halting as appropriate.