A navigation apparatus comprises a navigation-related information generating section and a display section. The navigation-related information generating section measures the position and orientation of an object and a target in a three-dimensional space and generate navigation-related information to be used for navigating the object toward the target. The display section displays the navigation-related information generated by the navigation-related information generating section in any of different modes depending on the relationship of the position and orientation of the object and that of the target. A surgical operation image acquisition/display apparatus comprises an observation section, an image display section and a specifying section. The observation section includes a plurality of observation sections whose position and orientation is modifiable. The image display section is adapted to alternatively display any of the images obtained by the observation sections or synthetically combine and display the combined images. The specifying section specifies the image to be displayed to the image display section according to the position and orientation of the observation section.