Provided is a positron emission tomography (PET) detector module using Geiger-mode avalanche photodiode (GAPD) as a photosensor. The PET detector module includes: a PET detector unit with a scintillation crystal detecting gamma rays emitted from a living body and converting them into a scintillation light and a first GAPD photosensor and a second GAPD photosensor each being connected to either end of the scintillation crystal and converting the scintillation light into an electrical signal; and a depth of interaction (DOI) decoding unit receiving the signals from the PET detector unit and comparing amplitude of the signals detected by the first GAPD photosensor and the second GAPD photosensor, thereby providing the depth information where the gamma rays are incident on the scintillation crystal (DOI). The disclosed PET detector module can provide improved energy resolution and additional DOI information while maintaining linearity.