A radiation therapy device controller adapted to update a computed tomography image data, for controlling a radiation therapy device that is adapted to irradiate 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 adapted to select, as computed tomography image data ( s D1') of an update target including the diseased portion the computed tomography image data of a set body motion phase (p1') from a computed tomography image data group ( s D1', s D2' s D3') generated for each body motion phase in advance; a radiation projection image generation unit adapted to rotate the ray source and the sensor array to generate a radiation projection image corresponding to each of a plurality of rotation angles, and to record the radiation projection image, the rotation angle when rotating the ray source and the sensor array at the time of generating the radiation projection image, and a body motion phase at the time of generating the radiation projection image to be correlated with each other; a rotation angle detection unit adapted to detect the rotation angle at the time of generating the radiation projection image; a reconstructed image generation unit adapted to generate a reconstructed image (F, k D1') when the computed tomography image data of the update target is modelled at the detected rotation angle; a difference information generation unit adapted to compare each pixel of the radiation projection image with its corresponding pixel of the generated reconstructed image to generate difference information indicating a luminance difference for the each pixel; a luminance update amount calculation unit adapted to identify a pixel on a straight line connecting the ray source and a detection element of the sensor array in the computed tomography image data of the update target, to calculate a luminance update amount candidate value for the each identified pixel based on an amount of the luminance change (S 1 (t)) of one pixel in a plurality of different computed tomography images and the difference information, and to calculate a luminance update amount of the each identified pixel using the luminance update amount candidate value of the each identified pixel calculated for a plurality of rotation angles corresponding to the body motion phase that is a target; and an updating unit adapted to update a luminance value of each corresponding pixel of the computed tomography image data of the update target, using the luminance update amount of the each identified pixel, wherein the computed tomography image is generated based on information detected by the sensor array. a computed tomography image selection unit adapted to select, as computed tomography image data ( s D1') of an update target including the diseased portion the computed tomography image data of a set body motion phase (p1') from a computed tomography image data group ( s D1', s D2' s D3') generated for each body motion phase in advance; a radiation projection image generation unit adapted to rotate the ray source and the sensor array to generate a radiation projection image corresponding to each of a plurality of rotation angles, and to record the radiation projection image, the rotation angle when rotating the ray source and the sensor array at the time of generating the radiation projection image, and a body motion phase at the time of generating the radiation projection image to be correlated with each other; a rotation angle detection unit adapted to detect the rotation angle at the time of generating the radiation projection image; a reconstructed image generation unit adapted to generate a reconstructed image (F, k D1') when the computed tomography image data of the update target is modelled at the detected rotation angle; a difference information generation unit adapted to compare each pixel of the radiation projection image with its corresponding pixel of the generated reconstructed image to generate difference information indicating a luminance difference for the each pixel; a luminance update amount calculation unit adapted to identify a pixel on a straight line connecting the ray source and a detection element of the sensor array in the computed tomography image data of the update target, to calculate a luminance update amount candidate value for the each identified pixel based on an amount of the luminance change (S 1 (t)) of one pixel in a plurality of different computed tomography images and the difference information, and to calculate a luminance update amount of the each identified pixel using the luminance update amount candidate value of the each identified pixel calculated for a plurality of rotation angles corresponding to the body motion phase that is a target; and an updating unit adapted to update a luminance value of each corresponding pixel of the computed tomography image data of the update target, using the luminance update amount of the each identified pixel, wherein the computed tomography image is generated based on information detected by the sensor array.