The present invention relates to an image sensor comprising an amorphous silicon thin-film transistor optical sensor which functions as an image sensor used for an X-ray photography device, a fingerprint recognition apparatus, a scanner, etc., and a method of manufacturing the image sensor. Since the thin-film transistor optical sensor according to the present invention has a high-resistance silicon region by disposing an offset region in a channel region, a dark leakage current of the optical sensor remains in a low level even under a high voltage. Therefore, it is possible to apply a high voltage to the thin-film transistor optical sensor according to the present invention so that the image senor can be sensitive to a weak light. In addition, since the storage capacitance in the image sensor is formed in a double structure, the image sensor has a high value of capacitance. Furthermore, since a lower common electrode is electrically connected to an upper common electrode, the image sensor has a stable structure.