A hadron therapy installation comprising: an irradiation unit with a horizontal axis of rotation capable of rotating around a treatment area between a first angular position (α), situated above said treatment area, and a second angular position (β), situated below said treatment area; a moving floor (8', 8") comprising a lower segment and an upper segment (8") situated on either side of said irradiation unit; a guide structure guiding said moving floor (8', 8") along a path around said treatment area such that it forms a substantially horizontal floor surface below said treatment area and a wall surrounding said treatment area as far as said first angular position (α) above said floor surface; said lower segment being able to be pulled by said irradiation unit from a lower docked position, in which it is found when said irradiation unit is in said second angular position (β), to a final operational position, in which it is found when said irradiation unit is in said first angular position (α), and in which it forms said substantially horizontal floor surface; and said upper segment (8") being able to be pulled by said irradiation unit from an upper docked position, in which it is found when said irradiation unit is in said first angular position (α), to a final operational position, in which it is found when said irradiation unit is in its second angular position (β), and in which it at least partially forms said substantially horizontal floor surface; wherein said lower segment and said upper segment (8") are segments with a finite length, each comprising a free end; characterised in that the moving floor is in the form of a band deformable in its longitudinal direction and in that a traction device (17, 19) is connected to said free end of said upper segment (8"), so as to exert a traction force on said upper segment toward said upper docked position sufficient to keep said upper segment under axial tension independent from the angular position and direction of movement of said irradiation unit. an irradiation unit with a horizontal axis of rotation capable of rotating around a treatment area between a first angular position (α), situated above said treatment area, and a second angular position (β), situated below said treatment area; a moving floor (8', 8") comprising a lower segment and an upper segment (8") situated on either side of said irradiation unit; a guide structure guiding said moving floor (8', 8") along a path around said treatment area such that it forms a substantially horizontal floor surface below said treatment area and a wall surrounding said treatment area as far as said first angular position (α) above said floor surface; said lower segment being able to be pulled by said irradiation unit from a lower docked position, in which it is found when said irradiation unit is in said second angular position (β), to a final operational position, in which it is found when said irradiation unit is in said first angular position (α), and in which it forms said substantially horizontal floor surface; and said upper segment (8") being able to be pulled by said irradiation unit from an upper docked position, in which it is found when said irradiation unit is in said first angular position (α), to a final operational position, in which it is found when said irradiation unit is in its second angular position (β), and in which it at least partially forms said substantially horizontal floor surface;