A particle beam irradiation chamber (1) comprises a passage (3) having a first opening portion (7) at an inside wall and a second opening portion (8) at an outside wall, and has an isocenter (6) within the chamber, wherein a first segment (9), which connects the center of the first opening portion (7) to the center of the second opening portion (8), passes through the inside of the passage (3), and the angle between a second segment (10), which connects the center of the first opening portion (7) to the isocenter (6), and the first segment (9) is set to be less than 180 degree, such that the width of the passage (3) is smaller than the opening width of the first opening portion (7).粒子線照射室(1)係具備通路(3),該通路(3)具有內壁側之第1開口部(7)與外壁側之第2開口部(8),而在室內具有等角點(6),其中,使連結第1開口部(7)的中心與第2開口部(8)的中心之第1線段(9)係通過通路(3)的內部,並將連結第1開口部(7)的中心與等角點(6)之第2線段(10)與第1線段(9)所成之角度設為180度以下,且使通路(3)的寬度小於第1開口部(7)的開口寬度。1‧‧‧粒子線照射室2‧‧‧遮蔽壁3‧‧‧通路4‧‧‧治療台5‧‧‧粒子線照射噴嘴6‧‧‧等角點7‧‧‧第1開口部8‧‧‧第2開口部9‧‧‧第1線段9a‧‧‧第1向量9b‧‧‧第2向量10‧‧‧線段P、Q‧‧‧分量
