A PET apparatus includes an optical coupling detachment testing unit. In one example, the optical coupling detachment testing unit inputs an electric signal to a piezoelectric element or the like adhered to a detector module and generates a sound wave within the detector module. Further, the optical coupling detachment testing unit detects the sound wave propagated within the detector module and performs a frequency analysis on the detected sound wave. Subsequently, as a result of the analysis, the optical coupling detachment testing unit detects whether an optical coupling detachment has occurred by looking for a frequency distribution specific to a surface having an optical coupling detachment and/or comparing a frequency distribution with another frequency distribution from a previous test.