The present invention provides an apparatus for measuring a Depth-Of-Interaction (DOI), comprising a crystal layer 10 of a mono layer in which a plurality of crystals for absorbing gamma rays are consecutively arranged, scintillation light detectors disposed at one end of the crystals and configured to detect scintillation light emitted from the crystal layer 10 by the gamma rays, change means included in the crystals and configured to linearly change transmittance in a length direction of the crystals, and a control unit 30 configured to calculate the DOI in the crystal layer 10 on a basis of the first output signal and the second output signal. The scintillation light detector outputs the first output signal in one direction and the second output signal in a direction at a right angle to the one direction.