This radiation therapy device controller selects, for a plurality of body motion phases, computed tomography image data obtained which shows a diseased portion, and generates, for each rotation angles of a ray source and a sensor array, a reconstructed image. The control device generates a radiation projection image which shows the diseased portion when the radiation is radiated if the rotation angle is a predetermined rotation angle, compares the reconstructed image of each of the plurality of body motion phases with the radiation projection image, and determines a body motion phase indicated by a reconstructed image in which a difference is small, to be a current respiratory phase. As a result, a position of the diseased portion calculated for the computed tomography image data in the computed tomography image data group of the respiratory phase in advance is identified to be a current position of the diseased portion.