An image data processing device including: a storage means configured to store a three-dimensional MRI image of a subjects head a three-dimensional appearance image generation means configured to generate a three-dimensional appearance image of the head an image generation means configured to perform positional alignment between the three-dimensional MRI image and the three-dimensional appearance image, and to generate a three-dimensional image of the head after the positional alignment an after-movement image generation means configured to generate, when the subjects head has been moved, a three-dimensional subjects head image after the movement and the positional alignment an operational object image generation means configured to generate an operational object image indicating a position of an operational object and a display means configured to display the three-dimensional image of the subjects head after the movement and the operational object image in one image.
