A scintillator array including a plurality of scintillators, an optical detector array corresponding to the scintillators, an AD conversion unit configured to convert an analog signal output from each optical detector into digital data, and a position detection processing unit configured to specify a position of the scintillator on which the radiation is incident are provided. If there are two different pieces of digital data at the same time, the position detection processing unit determines that radiation is incident on two scintillators when energy value data of the two pieces of digital data is greater than an energy value of a Compton edge and specifies the address of the scintillator on which the radiation is incident by comparing the energy values of the two pieces of digital data when at least one of the two energy values is less than the energy value of the Compton edge.