It is an object to improve the collimation performance or focusing performance of a grid collimator with respect to the same point. A collimator includes a collimator frame (16) having a shape corresponding to part of a circular ring, a plurality of first partition plates (31) which are supported on the collimator frame, are radially arrayed along a circumferential direction of the circular ring, and have shielding properties with respect to radiation, a plurality of first guide grooves (35) radially provided in a surface of each of the first partition plates, and a plurality of second partition plates (33) which are supported in the plurality of first guide grooves, are radially arrayed respectively in gaps between the first partition plates, and have shielding properties with respect to the radiation.
