The invention is intended to shorten a positioning time required for forming
an irradiation area with high accuracy using a number of leaf plates, and to reduce
physical and mental burdens imposed on patients. A multi-leaf collimator comprises
leaf plate driving body each including a plurality of movable leaf plates and provided
respectively on one side and the other side, the plurality of leaf plates of the
leaf plate driver on one side and the plurality of leaf plates of the leaf plate
driver on the other side being disposed in an opposing relation to form an irradiation
field of a radiation beam between the opposing leaf plates. Each of the leaf plate
driving body includes a motor provided in common to the plurality of leaf plates.
Driving force of the motor can be transmitted to the plurality of leaf plates at
the same time through a pinion gear, upper and lower air cylinders, and upper and
lower guides. Also, the driving force can be cut off selectively for each leaf plate.