A positron emission tomography apparatus installs a plurality of detector units in the circumference of a bed. The detector unit installs a plurality of combined substrates including detectors, analogue ASICs, and a digital ASIC and a voltage adjustment device inside a housing. A partition plate installed inside the housing separates the region inside the housing into a first region installed with the combined substrates and a second region installed with the voltage adjustment device. The partition plate blocks noise generated in the voltage adjustment device so as not to affect .gamma.-ray detection signals outputted from the detectors, thereby preventing the effect of the noise generated in the voltage adjustment device toward .gamma.-ray detection signals and shortening the examination time.