Provided is a radiation therapy apparatus on which quality management is performed. The radiation therapy apparatus includes: a body unit； a gantry coupled to one side of the body unit and formed to be rotatable in at least one direction with regard to the body unit； a radiation irradiating head formed at one side of the gantry to irradiate a radiation； and a level control module formed at one side of the radiation irradiating head to measure a rotation angle of the gantry or the radiation irradiating head, compare the measured rotation angle with a rotation angle indicated by an angle indicator of the radiation therapy apparatus, and correct an error of the angle indicator.
