A particle beam irradiation chamber (1) in which a passage (3) having a first opening part (7) at a side of an inner wall and a second opening part (8) at a side of an outer wall is provided and which has an isocenter (6) inside the chamber, wherein a fist line segment (9) which connects the center of the first opening part (7) and the center of the second opening part (8) passes inside the passage (3), an angle, which is formed by a second segment (10), which connects the center of the first opening part (7) and the isocenter (6) and the first line segment (9), is smaller than 180°, and a width of the passage (3) is narrower than a width of opening of the first opening part (7).
