A photo sensor (200) that is capable of generating a photo sensing signal corresponding only to ambient light by comprehending changes in electrical current depending on the change of temperature and compensating for the electrical current according the change of temperature and a flat panel display device using the photo sensor. The photo sensor includes: a photo sensing unit (210) generating a first current corresponding to an ambient light and a second current corresponding to an ambient temperature; a temperature compensating unit (220) including a dark diode (DPD) generating a third current having a same magnitude as the second current, corresponding to the ambient temperature due to block of light to be incident; and a buffer unit (230) outputting a light sensing signal corresponding to current having the same magnitude as the first current by subtracting the third current generated in the temperature compensating unit from the second current generated in the photo sensing unit (210).