1. A pen device, comprising: a micro-switch; a light source indicating x-y coordinates of strokes, wherein when the pen device has a pen tip which, when the pen tip touches a writing surface with a pen pressure, the micro-switch is turned on to let the light source indicating x-y coordinates of strokes turn ON and generate a “light point”; a pressure sensor for converting the pen pressure of the pen tip to an electrical signal representing the pen pressure for output; an A/D converter for converting the electrical signal representing the pen pressure to digital data of pen pressure; a micro-controller, in response to an ON signal from the micro-switch, for periodically sampling the electrical signal representing the pen pressure outputted from the pressure sensor when writing with the pen device, and receiving the digital data of pen pressure converted by the A/D converter; and a plurality of light sources for the transmission of digital data of pen pressure for outputting the digital data of pen pressure.