A Nested Marker, a novel visual marker for camera calibration in Augmented Reality (AR), enables accurate calibration even when the observer is moving very close to or far away from the marker. Our proposed Nested Marker has a recursive layered structure. One marker at an upper layer contains four smaller markers at the lower layer. Smaller markers can also have lower-layer markers nesting inside them. Each marker can be identified by its inside pattern, so the system can select a proper calibration parameter set for the marker. When the observer views the marker close-up, the lowest layer marker will work. When the observer views the marker from a distance, the top-layer marker will work. It is also possible to simultaneously utilize all visible markers in different layers for more stable calibration. Note that Nested Marker can be used in a standard ARToolkit framework. We have also developed an AR system to demonstrate the ability of Nested Marker.