A semiconductor device includes a first semiconductor chip operating at a first power supply voltage and a second semiconductor chip operating at a second power supply voltage lower than the first power supply voltage to supply the second power supply voltage to the first semiconductor chip. The semiconductor chips according to the present invention are conveniently used for fabrication of the semiconductor device. The first semiconductor chip includes an output circuit including a first transistor and a second transistor, interconnected in series and turned on or off complementarily. The output circuit outputs a signal to a first external output terminal. The first semiconductor chip also includes a third transistor connected in series with the first and second transistors and having a gate electrode connected to a second output terminal. The entire chip area is reduced, as compared with the case where plural semiconductor chips, operated at different operating voltages, are interconnected and used as such in a semiconductor device provided with an input/output buffer operating at a voltage different from the respective operating voltages resulting in an increased chip area.