When calculating a system matrix (detection probability), adjustment is made by fitting a point spread function (PSF) expressed by a Gauss 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 of the above PSF is adjusted for each layer in a depth direction of gamma-ray detectors indicating an incident direction in which the radiation strikes. By calculating the system matrix (detection probability) after adjusting the distance range of the function for each layer, improvement can be made in image quality of a reconstructed image.