1. A radiation imaging apparatus that moves a radiation image detecting means that obtains images of subjects by receiving irradiation of radiation which has passed through the subject with respect to the subjects to obtain a plurality of images at different positions, and combines the plurality of obtained radiation images to obtain elongated images having greater dimensions than the detectable range of the radiation image detecting means, comprising: a radiation source for irradiating radiation; a laser source for emitting a laser beam; displacement measuring means, for measuring distances to a measurement target by receiving the laser beam which is reflected by the measurement target; the radiation image detecting means; moving means, for moving the radiation image detecting means in a direction along and parallel to a radiation detecting surface of the radiation image detecting means; image processing means, for generating the elongated images by combining the plurality of radiation images; and control means, for controlling the laser source, the displacement measuring means, and the image processing means such that the laser source scans positions that correspond to the end portions of images in the movement direction of the radiation image detecting means in the direction perpendicular to the movement direction with the laser beam at each of a plurality of imaging operations, the displacement measuring means measures the positions of the end portions of the subjects in the scanning direction of the laser beam by receiving the laser beam which is reflected during the scanning, and the image processing means matches the combining positions of the radiation images such that the end portions of the subjects which are measured during each imaging operation are matched, to generate the elongated images.