1. A two-dimensional position map correcting method used when detecting radiation with radiation detectors each formed of a plurality of scintillator elements arranged in one dimension, two dimensions or three dimensions, and a light sensor optically coupled thereto, for preparing a look-up table from a two-dimensional position map presenting, in two dimensions, signal strengths obtained with the light sensor as corresponding to incident positions of the radiation incident on the scintillator elements, the two-dimensional position map correcting method comprising a first adding step for adding the signal strengths in a portion of the two-dimensional position map, wherein respective boundaries are drawn based on results of addition in the first adding step, and the two-dimensional position map is corrected based on signal strengths enclosed by these boundaries and wherein the first adding step is executed to add the signal strengths of each portion of the two-dimensional position map; the method comprising a boundary drawing step for drawing the respective boundaries by comparing results of addition of the each portion in the first adding step; a second adding step for adding the signal strengths at the respective boundaries; an area setting step for setting one reference area based on results of addition in the second adding step; a delimiting reference point, determining step for determining respective delimiting reference points based on respective signal strengths in the reference area set in the area setting step; and a boundary redrawing step for redrawing respective boundaries along the respective delimiting reference points determined in the delimiting reference point determining step; the two-dimensional position map being corrected based on signal strengths enclosed by the boundaries redrawn in the boundary redrawing step.