There are provided a leaf row in which a plurality of leaf plates are arranged in the thickness direction of the row in such a way that the respective one end faces of the leaf plates are trued up and a leaf plate drive mechanism that drives each of the plurality of leaf plates in such a way that the one end face approaches or departs from a beam axis. In each of the leaf plates, a facing side facing a leaf plate that is adjacent to that leaf plate in the thickness direction is formed of a plane including a first axis on the beam axis the leaf plate drive mechanism drives the leaf plate along a circumferential orbit around the second axis, on the beam axis, that is perpendicular to the beam axis and the first axis.
