There is provided a particle beam irradiation system so as to provide the dose distribution having more accuracy. An irradiation control part (13) comprises an energy setting controller (14) that sets the energy of a charged particle beam, a beam scanning controller (16) that controls a beam scanner (41), and a beam diameter changer (15) to control a beam diameter changer (40, 60), wherein the irradiation control part (13) sets a beam diameter of the charged particle beam to be a first beam diameter by the beam diameter changer (15), the charged particle beam is scanned step-wise by the beam scanning controller (16) so as to irradiate the charged particle beam on a predetermined region of the irradiation target, after that, the beam diameter of the charged particle beam is set by the beam diameter controller (15) to be a second beam diameter that is different from the first beam diameter, and the charged particle beam is scanned step-wise by the beam scan controller (16) so as to control the charged particle beam to irradiate on a region that is overlapped with at least a part of the predetermined part of the irradiation target.本發明的目的為在粒子束照射系統中,給予更佳精確度的高劑量分佈。照射控制部(13)係具備有:能量設定控制器(14),用以設定帶電粒子射束的能量;射束掃描控制器(16),用以控制射束掃描器(41);以及射束直徑控制器(15),用以控制射束直徑變更器(40、60);而照射控制部(13)係以下述方式進行控制:藉由射束直徑控制器(15)將帶電粒子射束的射束直徑設定為第一射束直徑,並藉由射束掃描控制器(16)將帶電粒子射束以階梯狀掃描並對照射目標的預定區域照射帶電粒子射束;之後,藉由射束直徑控制器(15)將帶電粒子射束的射束直徑設定為與第一射束直徑相異的第二射束直徑,並藉由射束掃描控制器(16)將帶電粒子射束以階梯狀掃描,並以對與照射目標的預定區域的至少一部份重疊的區域照射帶電粒子射束。S1、S2...圓TV...照射目標TVd...最深層
