A measurement area selection circuit has an irradiation field determination unit, an object area determination unit, and a measurement area determination unit. The irradiation field determination unit determines an irradiation field of an imaging surface of an FPD. The object area determination unit determines an object area from a comparison result between a first expected received dose of a directly exposed area and dose detection signals of detection pixels situated in the irradiation field. The measurement area determination unit determines a measurement area, which corresponds to a region of interest, from a comparison result between a second expected received dose of the measurement area and the dose detection signals of the detection pixels situated in the irradiation area and the object area. The dose detection signals of the detection pixels situated in the measurement area are used for AEC.