Radiological image detecting device, operating method therefor, and radiological imaging system

When a cumulative dose is predicted and automatic exposure control performed, the calculation of the prediction time for the cumulative dose to reach the target dose is calculated in a short period of time. Detection pixels (38b) are provided in a detection panel (30) for detecting X-rays. A signal processing circuit (45) samples the dosage signals indicating the dose of X-rays per unit of time on the basis of output from the detection pixels (38b). An exposure start determining unit (52) determines whether or not X-ray exposure has started on the basis of the results of a comparison of the dosage signals to the exposure start threshold value. An AEC unit (53) measures, on the basis of these dosage signals, the cumulative dose from the time (T0) at which X-ray exposure started to the time (T1) at which a predetermined period of time has elapsed. The prediction time (TP), which predicts when the cumulative dose will reach a predetermined target dose, is calculated based on the measured dose. When the prediction time (TP) is reached, a stop exposure signal is sent to the radiation source control device (11), and X-ray exposure is stopped.