PURPOSE: To prevent an apparatus from increase in size and to form an irradiation field conformable to the shape at a deseased part by preparing at least a pair of collimaters by combining movably in the opening and closing directions a plurality of collimater structures divided in the width direction and communicating collimater structures of each group to a common link.

CONSTITUTION: A component in the direction X in an irradiation field is formed with a pair of collimaters 21A and 21B and is moved at a specified amt. by each driving apparatus 23 provided each correspondingly. Then, an outer part in the direction Y is formed with collimater plates 28 and 28 of a pair of multi- divided collimaters 22A and 22B and the collimater plates 28 and 28 of each group are moved through a rack 27 by means of each driving apparatus 23 and the collimaters 28 and 28 connected with linking bodies 29 and 29 move in the direction Y. An irradiation field corresponding to a deseased part is thus formed with collimaters 21A and 21B and multi-divided collimaters 22A and 22B.