A signal detector detects signals transmitted from signal generators disposed in a radiation detecting cassette, and a distance calculator calculates the distance between a radiation source and a radiation detector based on the signals detected by the signal detector. A determining unit determines whether the detected distance matches a predetermined distance from the radiation source to the radiation detector when a radiation image is captured. If the detected distance does not match the predetermined distance, the determining unit outputs, to a warning unit, a warning signal indicating that the detected distance does not match the predetermined distance, and also outputs a control signal for equalizing the detected distance with the predetermined distance, to a radiation source movement controller.