Provided is a radiation detecting device, including: a scintillator which emits light when radiation is irradiated thereto; and a photosensor array having light receiving elements for receiving the emitted light which are two-dimensionally arranged, in which: the scintillator has a phase separation structure for propagating the light emitted inside the scintillator in a light propagating direction, the phase separation structure being formed by embedding multiple columnar portions formed of a first material in a second material; the radiation is irradiated to the scintillator from a direction which is not in parallel to the light propagating direction; and the light emitted inside the scintillator is propagated through the scintillator in the light propagating direction and is received by the photosensor array which is placed so as to face an end face of the scintillator.