A radiation detector is moved along a predetermined axis of movement. Radiation which has passed through a subject is irradiated onto the radiation detector each time the position of the radiation detector changes due to the movement. Signals are read out from the radiation detector each time that radiation is irradiated, to obtain a plurality of radiation images of the subject. Mechanical errors of the radiation detector are detected at each imaging operation, and mechanical error corrected images in which the mechanical errors are corrected are obtained. Amounts of shift of the subject among the mechanical error corrected images are detected, and amounts of movement of the subject during imaging operations are detected based on the amounts of shift.