A sensor panel of an electronic cassette includes detection pixels each for outputting a dose signal corresponding to a dose of X-rays, an irradiation start judging section for judging whether or not X-ray irradiation has been started based on the dose signal, and an AEC section for judging whether or not an accumulated dose of X-rays has reached a target dose based on the dose signal. A gain setting section sets a gain of an integration amplifier in the case of using the irradiation start judging section lower than that in the case of using the AEC section.
