1. A radiographic image detection apparatus comprising: a radiation source including a collimator that determines the irradiation range of radiation that is output to a subject; a radiographic image detector that detects, as a radiographic image, the radiation that has been output from the radiation source and transmitted through the subject; a detector movement means that moves the radiographic image detector parallel to a radiation detection surface; an imaging control means that controls the collimator of the radiation source so that a collimator image that extends along the movement direction of the radiographic image detector is included in the radiographic image; an image correction means that corrects the inclination of the radiographic image based on the collimator image in the radiographic image detected by the radiographic image detector; and an image synthesis means that produces a synthesis image by combining a plurality of radiographic images corrected by the image correction means.