1. An image data processing device comprising: a storage means configured to store a three-dimensional MRI image of a subject's head that has been previously taken; a three-dimensional appearance image generation means configured to generate a three-dimensional appearance image of the subject's 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 subject's head after the positional alignment; an after-movement image generation means configured to generate, when the subject's head has been moved, a three-dimensional image of the subject's head after the movement and the positional alignment; an operational object image generation means configured to generate an image of an operational object indicating a current position of the operational object operated and moved so as to maintain a positional relation with a specific portion on the three-dimensional MRI image of the subject's head; and a display means configured to display the three-dimensional image of the subject's head after the movement and the image of the operational object in one image.