A control unit of an electronic cassette has a function of switching a power supply state to an ADC between an operating state corresponding to a first state in which first power is supplied and a non-operating state corresponding to a second state in which second power that is lower than the first power per unit time is supplied. The control unit reduces the number of ADCs in the operating state per unit time T in an AED operation for detecting the start of the emission of X-rays to be less than that in an image reading operation for outputting an X-ray image provided for diagnosis.