A positron CT scanner includes a plurality of detectors arranged in a generally cylindrical multi-layers of rings, and slice septa for restricting visual fields of the detectors so that the visual fields intersect a set axis. An interval of the slice septa is substantially corresponds to a plural number of detector cells each of which can determine positions of .gamma. rays incident in the direction of the set axis. The slice septa are movable relatively to the detector cells in the direction of the set axis during a .gamma. ray counting operation.