Plural camera heads are each placed in a position capable of detection of radiation emitted from a radiation source. Compton cones obtained from the detection data output from each of the plural camera heads are projected onto three-dimensional space and an image based on the radiation source is reconstructed. In addition, projected images from projecting Compton cones obtained from the detection data output from each camera head onto two-dimensional planes are also employed for determining a three-dimensional space excluding regions where the radiation source is not present. Reconstruction of images based on the radiation source is then performed within the determined three-dimensional space.