Abstract: A detecting unit is formed by disposing 2-dimensional position sensitive type radiation detectors in the form of a plurality of layers superposed on each other, each capable of determining position of each of the reactions in multiple Compton scatterings produced by a .gamma.-ray in the detecting unit and the energy which the .gamma.-ray loses there are measured with a high precision. For each of the reactions it is examined whether the energy and the momentum conservation laws are satisfied or not in order to estimate probable sequences of reactions and thus a first and a second reactions are identified and the direction of the incident .gamma.-ray is presumed. The measurement precision is further improved by assigning the energy of the incident .gamma.-ray presumed from the reactions to the energy of a line spectrum.