1. A radiographic imaging system for acquiring a long radiographic image by combining short images taken in a plurality of imaging positions, the system comprising: imaging means including a radiation source for emitting radiation and a radiation detector for detecting radiation emitted from the radiation source and having penetrated a subject; moving means for moving the imaging means between a plurality of imaging positions that partially share an imaging region with each other; control means for sequentially moving the imaging means to the imaging positions with the moving means, causing the radiation source to emit radiation to acquire a short image with the radiation detector in each of the imaging positions, and acquiring a dark image with the radiation detector after acquiring the short image without emitting radiation from the radiation source; and image processing means for performing residual image correction upon the short image acquired in each of the imaging positions based upon the dark image acquired immediately before the acquisition of the short image and thereafter combining corrected short images to obtain a long radiographic image.