A radiation imaging device includes a radiation source which rotates around a subject to be examined and irradiates the subject with radiation, a two-dimensional detector in which a plurality of detection elements for detecting radiation from the radiation source are two-dimensionally arranged, and a grid which is placed in front of the two-dimensional detector to remove scattered radiation of the radiation. This device performs computation with respect to a subject image obtained by the two-dimensional detector by using the subject image and a gain image obtained by the two-dimensional detector, and executes removal of a grid pattern due to the grid and gain correction. The device reconstructs an image on the basis of the subject image obtained using the subject image from which a grid pattern due to the grid is removed and which is gain-corrected.