Focusing on a gamma ray detection phenomenon (event) in which a gamma ray from a gamma ray source is Compton scattered at a first-stage detector, the gamma ray is photoelectrically absorbed at a second-stage detector, the spatial distribution of the gamma ray source is imaged within a predetermined image space on the basis of measurement data for the interaction of the detectors and gamma rays. At this time, a probability parameter (vij) indicating the probability that Compton-scattered gamma ray arrived from within the image space and a detection sensitivity parameter (sij) indicating gamma ray detection sensitivity are set for each event and each pixel on the basis of the measurement data for each event, and these parameters are used to determine the pixel values (λj) for each pixel.