A flat panel X-ray detector which includes an X-ray-charge conversion film converting incident X-rays into electric charges, and a pair of electrodes disposed in contact with both surfaces of the X-ray-charge conversion film The X-ray-charge conversion film has a laminate structure including a plurality of metal halide films differing in band gap from one another and laminated along direction of c-axis of hexagonal crystal structure, and halogen atoms contained in the plurality of metal halide films are of the same kind among them.