To provide a radiation image reading apparatus and an X-ray imaging system with which X-ray transmission information of a specimen is obtained over a dynamic range.

A photomultiplier tube 110 is rotated within a light-receiving plane, a relative angle to a light guide 111 is varied, and a detection sensitivity distribution on a scanning line is measured for each relative angle. The variational width of the detection sensitivity is calculated from the detection sensitivity distribution. The relative angle, whose variation width is minimum, is determined as the optimum angle, and the lightguide 111 and the photomultiplier tube 110 are set at an optimum angle. Accordingly, signal change in areas other than the area, corresponding to X-ray transmission information received by the photomultiplier tube 110, is minimized whereas the signal change of the X-ray transmission information area is obtained in a dynamic range. In the end, accurate image information contained in the X-ray transmission information area is also digitized, which makes it possible to obtain a less nonuniform image that has uniformly high quality.