An X-ray detector includes an X-ray photoelectric conversion layer configured to produce electric charges in proportion to X-ray irradiation incident on the layer, a collecting electrode configured to collect the electric charges produced by the X-ray photoelectric conversion layer, a common electrode disposed on a surface of the X-ray photoelectric conversion layer opposite to the collecting electrode, a storage capacitor configured to store the electric charges collected by the collecting electrode, and a readout unit configured to read out the electric charges stored in the storage capacitor. A voltage is to be applied between the collecting electrode and the common electrode. The X-ray photoelectric conversion layer is formed of a polycrystalline oxide.