A direction finding radiation detector for detecting a direction of incidence of radioactive rays, the detector comprising: three or more scintillators (41, 42, 43, 44), formed by splitting a single scintillator in the circumferential direction and being arranged to overlap circumferentially, such that: the scintillators are shadowed by each other from radioactive rays incident in circumferential directions; the light emitted from one of the scintillators is not incident on the other scintillators; and the direction of incidence is detected all around in the range of 0° to 360°, photoreceptor devices (51, 52, 53) each having a light receiving surface optically coupled to each of the scintillators, wherein a combination of proportions of radioactive rays incident directly on the respective scintillators and radioactive rays incident indirectly, being shadowed by the other scintillators, varies with the direction of incidence circumferentially, characterized in that the scintillators are connected to any one of: (a) amplifiers (71, 72, 73) having respective different gains, the amplifiers are connected to a single A/D converter, and the A/D converter is connected to a pulse height analyzer corresponding to each of the scintillators; and (b) respective amplifiers (71, 72, 73), the amplifiers are connected to A/D converters having respective different conversion rates, and the A/D converters are connected to a pulse height analyzer corresponding to each of the scintillators, and the detector is connected to a computer that is configured to calculate a ratio R of counts of photoelectric peaks ascribable to the scintillators by calculating a sum total T and ratios R=(A/T, B/T, C/T), wherein A, B and C are counts of the photoelectric peaks. three or more scintillators (41, 42, 43, 44), formed by splitting a single scintillator in the circumferential direction and being arranged to overlap circumferentially, such that: the scintillators are shadowed by each other from radioactive rays incident in circumferential directions; the light emitted from one of the scintillators is not incident on the other scintillators; and the direction of incidence is detected all around in the range of 0° to 360°, photoreceptor devices (51, 52, 53) each having a light receiving surface optically coupled to each of the scintillators, wherein a combination of proportions of radioactive rays incident directly on the respective scintillators and radioactive rays incident indirectly, being shadowed by the other scintillators, varies with the direction of incidence circumferentially, characterized in that the scintillators are connected to any one of: (a) amplifiers (71, 72, 73) having respective different gains, the amplifiers are connected to a single A/D converter, and the A/D converter is connected to a pulse height analyzer corresponding to each of the scintillators; and (b) respective amplifiers (71, 72, 73), the amplifiers are connected to A/D converters having respective different conversion rates, and the A/D converters are connected to a pulse height analyzer corresponding to each of the scintillators, and (a) amplifiers (71, 72, 73) having respective different gains, the amplifiers are connected to a single A/D converter, and the A/D converter is connected to a pulse height analyzer corresponding to each of the scintillators; and (b) respective amplifiers (71, 72, 73), the amplifiers are connected to A/D converters having respective different conversion rates, and the A/D converters are connected to a pulse height analyzer corresponding to each of the scintillators, and the detector is connected to a computer that is configured to calculate a ratio R of counts of photoelectric peaks ascribable to the scintillators by calculating a sum total T and ratios R=(A/T, B/T, C/T), wherein A, B and C are counts of the photoelectric peaks.