A multileaf collimator comprising: two leaf groups each includinga plurality of leaves that extend in a first direction, each leave including at least one slit (71, 72) extending in the first direction and formed so as to pass therethrough; a frame that supports the plurality of leaves so that the plurality of leaves are individually advanceable and retractable in a state where the plurality of leaves are placed side by side in a second direction perpendicular to the first direction, said two leaf groups being arranged within the frame so as to face each other with a central part of the frame in the first direction interposed therebetween; a plurality of shafts that extend in the second direction passing through said slits of the leaves and are provided to correspond to the plurality of leaves, respectively; characterised by: a plurality of driving units that are respectively connected to first ends of the plurality shafts in directions of axial lines and rotate the shafts around the axial lines; and advance and retraction driving sections that are provided at second ends of the shafts in the directions of the axial lines, contact portions on at least one of the upper side (71a, 72a) and the lower side (71b, 72b) of the slits (71, 72)of the leaves, and advance and retract the leaves in the first direction by the rotation of the shafts around the axial lines, wherein at least some of the plurality of shafts corresponding to leaves constituting the same leave group have the same shaft length as each other, and wherein the plurality of driving units corresponding to leaves constituting the same leave group are arranged so that the positions thereof in the second direction are shifted from each other according to the positions of the second ends of the plurality of shafts having the same length, wherein at least some of the plurality of driving units are supported via spacers with respect to the frame, and wherein the thicknesses of the spacers in the second direction are set in a plurality of steps according to the positions of the driving units in the second direction. two leaf groups each includinga plurality of leaves that extend in a first direction, each leave including at least one slit (71, 72) extending in the first direction and formed so as to pass therethrough; a frame that supports the plurality of leaves so that the plurality of leaves are individually advanceable and retractable in a state where the plurality of leaves are placed side by side in a second direction perpendicular to the first direction, said two leaf groups being arranged within the frame so as to face each other with a central part of the frame in the first direction interposed therebetween; a plurality of shafts that extend in the second direction passing through said slits of the leaves and are provided to correspond to the plurality of leaves, respectively; characterised by: a plurality of driving units that are respectively connected to first ends of the plurality shafts in directions of axial lines and rotate the shafts around the axial lines; and advance and retraction driving sections that are provided at second ends of the shafts in the directions of the axial lines, contact portions on at least one of the upper side (71a, 72a) and the lower side (71b, 72b) of the slits (71, 72)of the leaves, and advance and retract the leaves in the first direction by the rotation of the shafts around the axial lines, wherein at least some of the plurality of shafts corresponding to leaves constituting the same leave group have the same shaft length as each other, and wherein the plurality of driving units corresponding to leaves constituting the same leave group are arranged so that the positions thereof in the second direction are shifted from each other according to the positions of the second ends of the plurality of shafts having the same length, wherein at least some of the plurality of driving units are supported via spacers with respect to the frame, and a plurality of driving units that are respectively connected to first ends of the plurality shafts in directions of axial lines and rotate the shafts around the axial lines; and advance and retraction driving sections that are provided at second ends of the shafts in the directions of the axial lines, contact portions on at least one of the upper side (71a, 72a) and the lower side (71b, 72b) of the slits (71, 72)of the leaves, and advance and retract the leaves in the first direction by the rotation of the shafts around the axial lines, wherein at least some of the plurality of shafts corresponding to leaves constituting the same leave group have the same shaft length as each other, and wherein the plurality of driving units corresponding to leaves constituting the same leave group are arranged so that the positions thereof in the second direction are shifted from each other according to the positions of the second ends of the plurality of shafts having the same length, wherein at least some of the plurality of driving units are supported via spacers with respect to the frame, and wherein the thicknesses of the spacers in the second direction are set in a plurality of steps according to the positions of the driving units in the second direction.