An FPD has plural pixels arranged in two dimensions. The pixels include a short pixel directly connected to a signal line, and a comparative pixel connected to another signal line through a TFT. In irradiation detecting operation, a control section monitors a voltage signal from the short pixel. When the voltage signal is a predetermined threshold value or more, the control section detects the start of X-ray irradiation, and provisionally starts charge accumulation operation. Then, the control section calculates the difference in the voltage signal between the short pixel and the comparative pixel. When this difference is another threshold value or more, the detection of the start of X-ray irradiation is judged to be valid. When the difference is less than the threshold value, the detection is judged to be misdetection caused by shock noise. The charge accumulation operation is aborted, and the irradiation detecting operation is restarted.