A PET scanner in which detector rings are arrayed in a multilayered manner so as to oppose each other in the body axis direction is provided. In the PET scanner, a predetermined number of detector units, each of which is made up of a predetermined number of detector rings, are arrayed so as to give each other a clearance, and a first ring set in which the clearance is less than or equal to a mean value of widths of two detector units forming each clearance and a second ring set constituted with a predetermined number of detector units are arrayed apart so as to give a clearance which is less than or equal to a mean value of the width of the first ring set and that of the second ring set, thereby imaging a field-of-view including the clearance and continuing in the body axis direction to an entire length of the first ring set and that of the second ring set.