In a case where power P_C supplied to CA during the image reading operation is normal power PN_C, in an AED operation of detecting irradiation start of X-rays, a control unit of an electronic cassette causes at least one of non-detection CAs other than a detection CA that is the charge amplifier connected to a detection channel of a detection pixel for irradiation start detection, among the plurality of CAs connected to a MUX to be in a power saving state in which the supply power P_C is lower than the normal power PN_C during the image reading operation.