A radiation therapy device controller identifies a pixel on a straight line connecting a ray source and a sensor array, and calculates a luminance update amount candidate value for each identified pixel based on a ratio of a change amount for the pixel on the straight line indicating the living body to a sum of change amounts from a luminance value of a pixel corresponding to a correlated computed tomography image correlated with a computed tomography image of an update target of a luminance value of the identified pixel. Also, the control device calculates a luminance update amount of each identified pixel using the luminance update amount candidate value of each identified pixel calculated for a plurality of rotation angles, and updates the luminance value of each corresponding pixel of the computed tomography image of the update target using the luminance update amount of each identified pixel.