A radiation detecting apparatus is provided. The radiation detecting apparatus includes a pair of rails extending in a channel direction, a plurality of collimator modules provided in the pair of rails and arranged in the channel direction, each collimator module having a plurality of collimator plates arranged in the channel direction, and a plurality of detector modules provided on a radiation outgoing side of the collimator modules and arranged in the channel direction, wherein each of the collimator modules has a pair of alignment pins extending along an irradiation direction, wherein the rails include a surface of placement for each collimator module, the surface of placement formed with one of concave holes and grooves in which first ends of the alignment pins are fitted, and wherein each of the detector modules has one of concave holes and through holes in which second ends of the alignment pins are fitted.
