1. A signal correcting method for use in a radiation image read-out apparatus, wherein position matching processing is performed on radiation image signal components of a radiation image signal representing a radiation image, which radiation image signal components have been obtained from a plurality of line sensor constituting units in the radiation image read-out apparatus comprising: i) a line light source for linearly irradiating stimulating rays onto an area of one surface of a stimulable phosphor sheet, on which the radiation image has been stored, the stimulating rays causing the stimulable phosphor sheet to emit light in proportion to an amount of energy stored thereon during its exposure to radiation, ii) a line sensor for receiving light, which is emitted from the linear area of the one surface of the stimulable phosphor sheet exposed to the linear stimulating rays or from a linear area of the other surface of the stimulable phosphor sheet corresponding to the linear area of the one surface of the stimulable phosphor sheet, and performing photoelectric conversion of the received light, the line sensor comprising the plurality of the line sensor constituting units, each of which comprises a plurality of photoelectric conversion devices arrayed along a length direction of the linear area of the stimulable phosphor sheet, the plurality of the line sensor constituting units being arrayed along the length direction of the linear area of the stimulable phosphor sheet such that certain regions of adjacent line sensor constituting units overlap each other, iii) scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction different from the length direction of the linear area of the stimulable phosphor sheet, and iv) reading means for successively reading outputs of the line sensor in accordance with the movement in order to obtain the radiation image signal, which represents the radiation image, the method comprising the steps of: a) reading out a test image, in which edge patterns extending in a direction different from the length direction of the linear area of the stimulable phosphor sheet are embedded at positions corresponding to the overlapping regions of the line sensor constituting units, test image signal components of a test image signal, which represents the test image, being thereby acquired from each of the line sensor constituting units, and b) performing the position matching processing on the radiation image signal components of the radiation image signal representing the radiation image, which radiation image signal components have been obtained from the plurality of the line sensor constituting units, and in accordance with edge pattern signal components, which represent the edge patterns embedded in the test image and are contained in the test image signal components of the test image signal representing the test image.