The charged particle beam to an object to be irradiated to the irradiation target with a low-energy small beam size even. The particle beam irradiation apparatus of the present invention (58), a vacuum window of a vacuum duct (6, 7) forming a vacuum region through which the charged particle beam (1), a charged particle beam (1) is released from the vacuum region and (8), the charged particle beam (1) and a scanning magnet for scanning in a direction perpendicular to the beam axis (2,3), the charged particle beam (1) of the passing position and the position monitor which detects the beam size (5) a monitor device having a (67) covers the vacuum window (8), downstream to the monitor device (67) is a low scattering gas-filled chambers which are arranged as (39), and controls the irradiation of the charged particle beam (1) irradiation management device (20) comprises a low-scattering gas filling chamber (39), the relative position in the beam axis direction of the monitor unit (67) and the vacuum window (8) is changeable to a desired position , when the charged particle beam (1) is irradiated, low scattering gas scattering is smaller than the air I is characterized by being filled.荷電粒子ビームが低エネルギーであっても小さなビームサイズで照射対象に照射することを目的とする。本発明の粒子線照射装置(58)は、荷電粒子ビーム(1)が通過する真空領域を形成する真空ダクト(6,7)と、荷電粒子ビーム(1)が真空領域から放出される真空窓(8)と、荷電粒子ビーム(1)をビーム軸に垂直な方向に走査する走査電磁石(2,3)と、荷電粒子ビーム(1)の通過位置及びビームサイズを検出する位置モニタ(5)を有するモニタ装置(67)と、真空窓(8)を覆い、下流側にモニタ装置(67)が配置された低散乱ガス充填室(39)と、荷電粒子ビーム(1)の照射を制御する照射管理装置(20)と、を備え、低散乱ガス充填室(39)は、モニタ装置(67)と真空窓(8)とのビーム軸方向における相対位置が所望の位置に変更可能に配置され、荷電粒子ビーム(1)が照射される際に、空気よりも散乱が小さい低散乱ガスが充填されることを特徴とする。
