1. An X-ray detector comprising: a plurality of photo-sensing pixels, each of the photo-sensing pixels comprising a photodiode that generates an electrical detection signal corresponding to an incident light and a switching device that transmits the detection signal; a gate driver sequentially applying gate pulses, which turn on the switching device, to the switching device via a plurality of gate lines; and a read-out integrated circuit reading out the detection signal from the plurality of photo-sensing pixels, wherein the gate driver and the read-out integrated circuit read out a dark image used to estimate an image lag by an X-ray image after reading out the X-ray image obtained by an X-ray radiation.