A system comprising: a marker (10, 30, 210, 405, 505, 605) which is attachable to a target and configured so that a pattern surface (15, 250, 507, 607) formed inside the marker (10, 30, 210, 405, 505, 605) is visible through an optical system (12, 220, 506, 606) formed in an aperture (13, 230) of the marker (10, 30, 210, 405, 505, 605); and an optical tracking system for tracking a location and a posture of a the marker (10, 30, 210, 405, 505, 605), wherein the optical tracking system comprises a first image capturing part (110, 270, 410, 510, 610) configured to capture a first image (450, 550, 650) including a pattern image (320, 455, 655) in which a part of the pattern surface (15, 250, 507, 607) is identifiably captured, the first image capturing part (110, 270, 410, 510, 610) forming an infinite optical system together with the marker (10, 30, 210, 405, 505, 605), characterized in that the optical tracking system further comprises: a second image capturing part (120, 280, 420, 520, 620) configured to capture a second image (460, 560, 660) including an outgoing light image of an outgoing light emitted through the aperture (13, 230) in a first direction; a third image capturing part (130, 290, 430, 530, 630) configured to capture a third image (470, 570, 670) including an outgoing light image of an outgoing light emitted through the aperture (13, 230) in a second direction differing from the first direction, and a processor (140, 440, 540, 640) configured to determine the posture of the marker (10, 30, 210, 405, 505, 605) based on the first image (450, 550, 650), and to determine the location of the marker (10, 30, 210, 405, 505, 605) based on the second image (460, 560, 660) and the third image (470, 570, 670), 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 (10, 30, 210, 405, 505, 605). a marker (10, 30, 210, 405, 505, 605) which is attachable to a target and configured so that a pattern surface (15, 250, 507, 607) formed inside the marker (10, 30, 210, 405, 505, 605) is visible through an optical system (12, 220, 506, 606) formed in an aperture (13, 230) of the marker (10, 30, 210, 405, 505, 605); and an optical tracking system for tracking a location and a posture of a the marker (10, 30, 210, 405, 505, 605), wherein the optical tracking system comprises a first image capturing part (110, 270, 410, 510, 610) configured to capture a first image (450, 550, 650) including a pattern image (320, 455, 655) in which a part of the pattern surface (15, 250, 507, 607) is identifiably captured, the first image capturing part (110, 270, 410, 510, 610) forming an infinite optical system together with the marker (10, 30, 210, 405, 505, 605), characterized in that the optical tracking system further comprises: a second image capturing part (120, 280, 420, 520, 620) configured to capture a second image (460, 560, 660) including an outgoing light image of an outgoing light emitted through the aperture (13, 230) in a first direction; a third image capturing part (130, 290, 430, 530, 630) configured to capture a third image (470, 570, 670) including an outgoing light image of an outgoing light emitted through the aperture (13, 230) in a second direction differing from the first direction, and a processor (140, 440, 540, 640) configured to determine the posture of the marker (10, 30, 210, 405, 505, 605) based on the first image (450, 550, 650), and to determine the location of the marker (10, 30, 210, 405, 505, 605) based on the second image (460, 560, 660) and the third image (470, 570, 670), 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 (10, 30, 210, 405, 505, 605). a second image capturing part (120, 280, 420, 520, 620) configured to capture a second image (460, 560, 660) including an outgoing light image of an outgoing light emitted through the aperture (13, 230) in a first direction; a third image capturing part (130, 290, 430, 530, 630) configured to capture a third image (470, 570, 670) including an outgoing light image of an outgoing light emitted through the aperture (13, 230) in a second direction differing from the first direction, and a processor (140, 440, 540, 640) configured to determine the posture of the marker (10, 30, 210, 405, 505, 605) based on the first image (450, 550, 650), and to determine the location of the marker (10, 30, 210, 405, 505, 605) based on the second image (460, 560, 660) and the third image (470, 570, 670), 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 (10, 30, 210, 405, 505, 605).