An X-ray image detecting device has an FPD having a matrix of pixels each for accumulating signal charge in accordance with an X-ray irradiation amount. An imaging area of the FPD is partitioned into a plurality of divided sections A to I. Each of the divided sections A to I has a short pixel for detecting X-ray irradiation. In a synchronization control for controlling the FPD in synchronization with detection of a start of X-ray emission from an X-ray source, a control unit for controlling the X-ray image detecting device uses all the divided sections A to I. In an automatic exposure control for stopping the X-ray emission from the X-ray source by detecting a total X-ray irradiation amount, the control unit uses part of the divided sections, e.g. the short pixels of the divided sections that are judged to be opposed to an object in the synchronization control.