A portable computer system includes a portable computer equipped with a graphic chip, and an LCD monitor receiving a video signal from the graphic chip and displaying the video signal. The portable computer system further includes an external video signal input part provided at the portable computer and receiving an external video signal from an external computer, an A/D converter provided at the portable computer and converting the external video signal inputted through the external video signal input part into a digital signal, and a control part controlling the video signal and the digital signal from the graphic chip and the A/D converter to be outputted to the LCD monitor. With this configuration, the present invention provides a portable computer system enhancing the utility of the LCD monitor by receiving an external video signal from an external computer and displaying it on an LCD monitor.