An FPD, being offline from an X-ray source, detects X-ray irradiation from the X-ray source to detect an X-ray image. The FPD includes pixels arranged in two dimensions, scan lines corresponding to respective rows of the pixels, signal lines corresponding to respective columns of the pixels, and switching elements provided to the respective pixels to allow performing accumulation operation or readout operation. At least one of the pixels is used as a detection pixel to detect a start of the X-ray irradiation. First and second voltage signals are obtained successively through the signal line to which the detection pixel is connected. The start of the X-ray irradiation is judged based on a difference between the first and second voltage signals.