1. An imaging device using gamma rays comprising: a first Compton camera adapted to receive one gamma ray emitted from an imaging target to which a first probe having positron emitting nuclei and a second probe having gamma ray emitting nuclei are administered; a second Compton camera arranged opposite to the first Compton camera and adapted to receive another gamma ray emitted from the imaging target; an imaging processor to perform either a PET image reconstruction or a Compton image reconstruction depending on whether interactions with gamma rays have been detected by the first and the second Compton cameras or interactions with a gamma ray has been detected by either the first or the second Compton camera; and a display for displaying the PET image and the Compton image in association respectively with an accumulation region of the first probe and an accumulation region of the second probe, in a manner distinction can be made from each other.