1. A radiation image read-out method, comprising the steps of: i) linearly irradiating stimulating rays, which have been produced by a line light source, onto an area of one surface of a stimulable phosphor sheet, on which a 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) 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, with a line sensor comprising a plurality of pixel regions arrayed along a length direction of the linear area of the stimulable phosphor sheet, the received light being subjected to photoelectric conversion performed by the line sensor, iii) 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, iv) successively reading outputs of the line sensor in accordance with the movement, outputs of the pixel regions at respective positions of movement being thereby obtained, and v) obtaining a final image signal, which represents a final image, from the thus obtained outputs of the pixel regions at the respective positions of movement, wherein addition processing is performed on the outputs of a plurality of pixel regions constituting the line sensor, which pixel regions are adjacent to one another along the length direction of the linear area of the stimulable phosphor sheet, and which pixel regions have widths taken in the length direction of the linear area of the stimulable phosphor sheet such that a sum of the widths is equal to the width of one pixel in the final image, the width of the one pixel in the final image being taken in the length direction of the linear area of the stimulable phosphor sheet, an image signal component of the final image signal representing the final image, which image signal component represents one pixel in the final image, being thereby obtained.