Visualizing a deformation amount or displacement amount of a region of interest in therapy rendered in an image upon aligning images used in radiation therapy allows for enhancing an accuracy of alignment. In a positioning system 104, an image processing unit 201 calculates a deformation parameter using a non-rigid registration method for a region of interest having been set by a treatment planning device 101 based on a treatment plan CT image and three-dimensional tomographic image imaged for bed positioning and calculates a displacement amount parameter for a region extracted using a region extraction method. The image processing unit 201 calculates a positioning parameter representing a displacement amount of a bed based on the calculated deformation parameter and displacement amount parameter and displays the respective parameters together with an alignment image, thereby allowing for confirmation of the deformation amount and displacement amount.
