In an AED operation of detecting irradiation start of X-rays, a control unit of an electronic cassette selectively outputs the analog voltage signal from a part of the charge amplifiers including a detection CA connected to the detection channel of a detection pixel for irradiation start detection among the plurality of CAs connected to the MUX, to the ADC, causes the ADC to perform only the AD conversion process for the analog voltage signal selectively output from the multiplexer, and reduces the number of pulses NPU_A per unit time in a clock signal defining an operation timing of the ADC compared to that in the image reading operation.
