The present disclosure relates to an X-ray detector. The X-ray detector includes the first and second gate lines arranged to be spaced apart from each other on a substrate, a data line and a bias line that are arranged to be spaced apart from each other in a direction intersecting the first and second gate lines, and define a unit pixel area, a storage capacitor that is arranged in the unit pixel area and has one end connected to a ground, a phototransistor that is turned on by a reset signal applied to the first gate line and provides a signal generated by an incident light source to the storage capacitor, and a thin film transistor that is turned on by a gate signal applied to the second gate line to provide a charge stored in the storage capacitor to the data line.