A system comprising: a marker which is attachable to a target and configured so that a pattern surface formed inside the marker is visible through an optical system formed in an aperture of the marker ; and an optical tracking system for tracking a location and a posture of a the marker , wherein the optical tracking system comprises a first image capturing part configured to capture a first image including a pattern image in which a part of the pattern surface is identifiably captured, the first image capturing part forming an infinite optical system together with the marker , characterized in that the optical tracking system further comprises: a second image capturing part configured to capture a second image including an outgoing light image of an outgoing light emitted through the aperture in a first direction; a third image capturing part configured to capture a third image including an outgoing light image of an outgoing light emitted through the aperture in a second direction differing from the first direction, and a processor configured to determine the posture of the marker based on the first image , and to determine the location of the marker based on the second image and the third image , wherein the depths of field of the second and third images (460, 560, 660; 470, 570, 670) are formed in a predetermined finite range around the location of the marker . a marker which is attachable to a target and configured so that a pattern surface formed inside the marker is visible through an optical system formed in an aperture of the marker ; and an optical tracking system for tracking a location and a posture of a the marker , wherein the optical tracking system comprises a first image capturing part configured to capture a first image including a pattern image in which a part of the pattern surface is identifiably captured, the first image capturing part forming an infinite optical system together with the marker , characterized in that the optical tracking system further comprises: a second image capturing part configured to capture a second image including an outgoing light image of an outgoing light emitted through the aperture in a first direction; a third image capturing part configured to capture a third image including an outgoing light image of an outgoing light emitted through the aperture in a second direction differing from the first direction, and a processor configured to determine the posture of the marker based on the first image , and to determine the location of the marker based on the second image and the third image , wherein the depths of field of the second and third images (460, 560, 660; 470, 570, 670) are formed in a predetermined finite range around the location of the marker . a second image capturing part configured to capture a second image including an outgoing light image of an outgoing light emitted through the aperture in a first direction; a third image capturing part configured to capture a third image including an outgoing light image of an outgoing light emitted through the aperture in a second direction differing from the first direction, and a processor configured to determine the posture of the marker based on the first image , and to determine the location of the marker based on the second image and the third image , wherein the depths of field of the second and third images (460, 560, 660; 470, 570, 670) are formed in a predetermined finite range around the location of the marker . <EOS>