A method for assembling a collimator module, the collimator module including a plurality of first collimator plates arrayed in a first direction, each first collimator plate having a plurality of slots formed on a plate surface, and a plurality of second collimator plates arrayed in a second direction orthogonal to the first direction, wherein each second collimator plate penetrates respective slots along the first direction so as to form a lattice-shape, the method comprising:
- positioning the plurality of first collimator plates by moving a first collimator plate in one direction along the second direction, so that a side wall of a first cutout formed on an edge of an x-ray radiation incident side or an x-ray radiation output side of the first collimator plate contacts a member extending in the first direction.