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