A positron emission tomography "PET" apparatus (1100) according to an embodiment includes a PET detector (100), an acquisition unit (1170) and a calculation unit (1170). The PET detector (100) comprises a plurality of arrayed rings in each of which a plurality of detector crystals (105) are arrayed. The acquisition unit (1170) acquires a first crystal efficiency for each detector crystal (105) based on a measured gamma ray emitted from a line source, a second crystal efficiency for each detector crystal, which is calculated based on a gamma ray from a cylinder source, and a third crystal efficiency for each detector crystal, which is calculated based on a gamma ray from the line source. The calculation unit (1170) calculates a fourth crystal efficiency for each detector crystal based on the first crystal efficiency, the second crystal efficiency, and the third crystal efficiency.