A radiation therapy apparatus (20) includes: an annular gantry (30) an irradiation unit (24) which is provided on the gantry (30) and configured to radiate radiation a support (42A) which supports the gantry (30) a rotation-driving mechanism (40A) which is provided between the gantry (30) and the support (42A) and rotates the gantry (30) around a horizontal central axis (C1) of the gantry (30). The rotation-driving mechanism (40A) includes: annular rails (41) that are provided on the gantry (30) and sliding members (45A) which are provided on regions of the support (42A) that correspond to a lower half of the gantry (30), slidably guide the rails (41), and receive at least a load of the gantry (30) in a vertical direction.
