An X-ray detector includes a cassette including a first terminal and a second terminal, and a bucky including a first electrode and a plurality of second electrodes, the first terminal of the cassette being connected to the first electrode of the bucky and the second terminal of the cassette being connected to at least one of the plurality of second electrodes of the bucky, wherein the cassette and the bucky are configured to be connected to each other in at least two different connection modes, at least one of the second electrodes connected to the second terminal in a first connection mode being different from the second electrodes connected to the second terminal in a second connection mode.