A radiation therapy device controller for controlling a radiation therapy device that irradiates a living body arranged between a ray source and a sensor array with radiation from the ray source to treat a diseased portion of the living body, the controller comprising: a computed tomography image selection unit that selects, for each of a plurality of body motion phases (p1',p2', p3') of the living body, computed tomography image data ( s D1') from a computed tomography image data group ( s D1', s D2', s D3') generated for each of the plurality of body motion phases (p1',p2', p3') of the living body; a reconstructed image generation unit that generates, for each rotation angles of the ray source and the sensor array, a reconstructed image (F, k D1') corresponding to the body motion phase (p1') using the selected computed tomography image data ( s D1'); a radiation projection image generation unit that generates a radiation projection image which shows the diseased portion when the radiation is radiated from the ray source to the sensor array when the rotation angle (An) is a predetermined rotation angle; characterised by further comprising: a body motion phase determination unit that compares a luminance of a pixel of the reconstructed image of each of the plurality of body motion phases (p1', p2', p3') with a luminance of pixel of the generated radiation projection image, determines the reconstructed image that has the smallest value of a sum of luminance differences of all pixels, and determines a current body motion phase of the living body by the determined reconstructed image; and a diseased portion tracking processing unit that identifies the computed tomography image data ( s D1') in the computed tomography image data group by the current body motion phase of the living body, and determines a position of the diseased portion by the position in the identified computed tomography image data ( s D1'), which is determined with a predetermined method. a computed tomography image selection unit that selects, for each of a plurality of body motion phases (p1',p2', p3') of the living body, computed tomography image data ( s D1') from a computed tomography image data group ( s D1', s D2', s D3') generated for each of the plurality of body motion phases (p1',p2', p3') of the living body; a reconstructed image generation unit that generates, for each rotation angles of the ray source and the sensor array, a reconstructed image (F, k D1') corresponding to the body motion phase (p1') using the selected computed tomography image data ( s D1'); a radiation projection image generation unit that generates a radiation projection image which shows the diseased portion when the radiation is radiated from the ray source to the sensor array when the rotation angle (An) is a predetermined rotation angle; a body motion phase determination unit that compares a luminance of a pixel of the reconstructed image of each of the plurality of body motion phases (p1', p2', p3') with a luminance of pixel of the generated radiation projection image, determines the reconstructed image that has the smallest value of a sum of luminance differences of all pixels, and determines a current body motion phase of the living body by the determined reconstructed image; and a diseased portion tracking processing unit that identifies the computed tomography image data ( s D1') in the computed tomography image data group by the current body motion phase of the living body, and determines a position of the diseased portion by the position in the identified computed tomography image data ( s D1'), which is determined with a predetermined method.