Abstract: A radiation therapy apparatus includes a radiation irradiation unit 35, an X-ray imaging unit 36, a CT imaging device 37, a treatment planning device 38, and a controller 10. The controller 10 includes a CT image deformation amount calculation unit 11, a shape calculation unit 12, an irradiation region determining unit 13, an X-ray image deformation amount calculation unit 14, a position calculation unit 15, a template matching unit 16, a comparison unit 17, a correction unit 18, and an image processing unit 19, and by comparing the positions of a treatment target locus in respective breathing phases and the positions of the treatment target locus in corresponding ones of the respective breathing phases identified by the template matching unit 16, identifies the error values between the positions and the positions identified by the template matching unit 16 for each of parameters.