An X-ray imaging apparatus has an electronic cassette, in which an FPD device receives X-rays applied by an X-ray source to a body, and stores charge according to a radiation dose of the X-rays, to create an image. A control unit controls the FPD device. A radiation sensor detects the radiation dose to be used for controlling the FPD device. A magnet as fastening device secures the radiation sensor removably in a radiation path between the X-ray source and the body. Also, an evaluation unit recognizes a start and end of application of the X-rays according to a dose signal from the radiation sensor, for the control unit to control the FPD device. A flexible support arm is disposed between the radiation sensor and the magnet, for keeping the radiation sensor in a changeable position relative to the magnet.
