As for the radiotherapy device, as the radiation beam which is directed alongside the beam axis is formed, the block boresight which restricts the width of the beam selectively the multiple leaf boresight which restricts the width of the beam selectively the expedient which possesses the width in the beam axis and the 1st and 2nd direction which is crossed and, at least in 1st direction and, at least in 2nd direction and, it has. The block boresight, being the portable possible diaphragm inside and outside the beam, includes the diaphragm which possesses specified thickness in the direction of the beam axis which changes. The diaphragm has the front edge, the particular front edge has been allowed to have possessed bigger thickness than even one territory which at least is on back of the front edge. In addition, the diaphragm has been allowed to have possessed the projection territory which extends from that rear section, facing toward the front edge, the particular front edge at least has been allowed to have possessed bigger thickness than even one which is arranged in the side vis-a-vis the projection territory territory. In addition, these had could have been overturned the territory where it is not covered sufficiently by MLC which is moved dynamically. Being control means for the multiple leaf boresight, the control means which make the leaf of the multiple leaf boresight expand in order to cover the territory of the beam which is covered by the part where the diaphragm is thin relatively may be provided. This is simplified when it is below. When the projection territory has extended most from the part of the rear of the diaphragm, when the projection territory has extended to the front edge of the diaphragm, when the projection territory the true tsu it has become immediately, when the projection territory becomes the central territory of the diaphragm, and, when width of the projection territory it increases facing toward the front edge of the diaphragm, is.
