A semiconductor integrated circuit device including an I/O circuitry capable of low-voltage high-speed operation at low cost is provided. In the I/O circuitry, when an I/O voltage (for example, 3.3 V) is lowered to a predetermined voltage (for example, 1.8 V), portions causing a speed deterioration are a level conversion unit and a pre-buffer unit for driving a main large-sized buffer. In view of this, a high voltage is applied to a level up converter and a pre-buffer circuit. By doing so, it is possible to achieve an I/O circuitry capable of low-voltage high-speed operation at low cost.