1. A radiation image acquiring method, wherein composition processing is performed on a plurality of read-out images, each of which has been obtained from an image read-out operation performed by image read-out means for reading out an identical radiation image having been stored on a stimulable phosphor sheet, and wherein a single composite image, which represents the radiation image having been stored on the stimulable phosphor sheet, is thereby acquired, the method comprising the steps of: i) performing the image read-out operation with the image read-out means for obtaining each of the read-out images, such that an efficient pixel existence range of efficient pixels, which are capable of being read out within a read-out sensitivity range of the image read-out means, varies for different read-out images, and such that the efficient pixel existence range in each of the read-out images contains an overlapping area, which overlaps with the efficient pixel existence range in one of the other read-out images, ii) performing correction processing on the read-out images, which have the common overlapping area, for correcting pixel values of pixels in at least either one of the read-out images, such that the pixel values of the pixels in the overlapping area of one of the read-out images become identical with the pixel values of the pixels in the overlapping area of the other read-out image, and iii) performing the composition processing for composing a single image from the read-out images, which have been obtained from the correction processing, whereby the composite image is acquired.