In a positron CT device of this invention, a cross range calculation section determines a cross range of a coincidence LOR as a virtual line that connects two detectors for performing coincidence and a pixel, and a system matrix calculating section determines a system matrix by calculating elements in the system matrix within the cross range upon calculating of the system matrix. Thereafter, a reconstruction section creates a distribution image of positrons as an image based on the system matrix. Consequently, improvement in speed of image reconstruction may be realized.
