1. A positron CT apparatus having: a plurality of detectors for detecting radiation released from a positron-emitting drug given to a subject, and outputting electric signals; a coincidence counting circuit for detecting, based on the electric signals, that a simultaneous observation of the radiation is made in two of the detectors; a system matrix calculating device for calculating a system matrix based on outputs of the coincidence counting circuit; and a reconstructing device for generating a distribution image of the positrons as an image based on the system matrix; the positron CT apparatus comprising: a function adjusting device for making adjustment by fitting a point spread function to a profile of count values of radiation with respect to a distance from a point source which emits radiation of the same type as the positron-emitting drug; and a distance range adjusting device for adjusting a distance range of the function for each layer in a depth direction of the detectors indicating an incident direction in which the radiation strikes; wherein the system matrix is calculated based on the function with the distance range adjusted.