A multi-leaf collimator (60) includes a plurality of leaves (70) aligned in a thickness direction of the leaf, a driving device (90) which advances and retracts the leaves (70) with respect to radiation (Sr), and an identification mark (102) identifiable at least from a direction perpendicular to both the advancing and retracting direction of the leaves (70) and the thickness direction of the leaf(70) to detect positions of the leaves (70), wherein, when the plurality of leaves (70) are disposed in a preset reference position (P), gaps (d2) between adjacent identification marks (102) in the thickness direction are formed to be larger than gaps (d1) between adjacent leaves (70) in the thickness direction.
