There are provided a leaf row (5C) in which a plurality of leaf plates (5L) are arranged in the thickness direction of the row (5C) in such a way that respective end faces (EL) of the leaf plates (5L) are trued up, and a leaf plate drive mechanism (5D) that drives each of the plurality of leaf plates (5L) in such a way that the end face (EL) approaches or departs from a beam axis (XB). In each of the leaf plates (5L), a facing side (PL) facing a leaf plate that is adjacent to that leaf plate in the thickness direction is formed of a plane (Psa) including a first axis (Asa) on the beam axis (XB) the leaf plate drive mechanism (5D) drives the leaf plate (5L) along a circumferential orbit (OL) around the second axis (Asb), on the beam axis (XB), that is perpendicular to the beam axis (XB) and the first axis (Asa).
