Disclosed is an X-ray detector and a method of manufacturing the X-ray detector. A cushion layer is interposed between a scintillator panel for converting X-rays into visual light and a lower substrate formed with a thin film transistor, a photoelectric conversion device. Thus, the lower substrate and the scintillator panel can be bonded to each other such that no air layer can be introduced between the lower substrate and the scintillator panel. Therefore, no air layer is introduced, and thus, it is possible to prevent the efficiency of light incident to the photoelectric conversion device from being lowered. Further, the cushion layer is employed such that the lower substrate and the scintillator panel can be separated and reused.