An emitted light in a scintillator element is sufficiently diffused in the scintillator array to be inputted into a photo multiplier tube (PMT) using a side face light guide that is optically coupled with respect to a side face of a scintillator array, except for in an end area. In the end area, the emitted light in the scintillator element is sufficiently diffused also in the side face light guide to be inputted into the PMT. In this way, also in the scintillator element in the end area, the emitted light is sufficiently diffused in the side face light guide, and thereby the precision of separation of a position calculation map in the end area may be improved, resulting in improved discriminating ability of a position in the end area.