A radiation detecting apparatus of this invention includes an arithmetic processing device which has a first adding step for adding signal strengths in one portion of a two-dimensional position map, draws respective boundaries based on results of addition in the first adding step, and corrects the two-dimensional position map based on signal strengths enclosed by these boundaries. The signal strengths of one portion of the two-dimensional position map, and not all the areas of the two-dimensional position map, are added. As a result, incident positions can be discriminated efficiently, and radiation detecting positions can be determined efficiently.