In an X-ray imaging apparatus, a detection panel has monitor pixels for monitoring X-rays. A signal processor samples a dose signal of a dose per unit time of X-rays according to an output of the monitor pixels. A start detector checks whether irradiation of X-rays is started according to a result of comparison between the dose signal and a start threshold. An AEC device acquires cumulative dose from a start time of the start of irradiation of X-rays until acquisition time after a predetermined time according to the dose signal. According to the cumulative dose, a predicted time point of a reach of the cumulative dose to a target dose is estimated. A stop signal is transmitted to a radiation source controller at the predicted time point, to stop the irradiation of X-rays.