<；P>；PROBLEM TO BE SOLVED: To provide a multileaf collimator for a particle beam treatment system which can detect the operation abnormality of leaves only by an inexpensive incremental encoder. <；P>；SOLUTION: When a leaf 8L is opened and the torque value of a motor 3L calculated by torque calculating means exceeds a predetermined value over a predetermined time, the contact of the leaf 8L with a stopper 7L is detected to clear a pulse integrated value 12P obtained by integrating the pulses produced by the encoder 5L. The leaf 8L is again brought into contact with the stopper 7L to determine the abnormality of a leaf driving device 30 depending on whether the pulse integrated value is in the vicinity of 0. <；P>；COPYRIGHT: (C)2010,JPO&INPIT
