The beam diameter of the charged particle beam is reduced to become smaller than the target beam diameter by the quadrupole electromagnets 2g, 2h, 2i according to the predetermined beam diameter reducing coefficient c, then the beam diameter which has been reduced to become smaller than the target beam diameter is enlarged by using the scatterer 8 arranged on the downstream side of the scanning electromagnets 7a, 7b to the target beam diameter, thereby the beam diameter is easily enlarged.藉由四極電磁鐵2g、2h、2i將帶電粒子射束的射束直徑按照預定的射束直徑縮小係數c縮小成比目標的射束直徑更小之後，使用掃描電磁鐵7a、7b的下游側具備的散射體8，將已縮小成比目標的射束直徑更小之帶電粒子射束的射束直徑擴大至目標的射束直徑，而簡便地使射束直徑穩定化。
