Described embodiments provide an X-ray detector and a method for driving the same. The X-ray detector includes: a sensor panel in which a plurality of pixels are defined, the plurality of pixels each including a photodiode for converting light corresponding to incident X-ray into an electric signal, and a switching element connected to one terminal of the photodiode to control the output of the electric signal; a light emitting unit for providing light to the photodiode; and a voltage supply unit connected to the other terminal of the photodiode to selectively supply first and second voltages different from each other.