1. A method of tracking augmented reality content, comprising: recognizing an augmented reality marker from an input image and calculating a position and posture of the recognized augmented reality marker; calculating a position and posture of augmented reality content corresponding to the augmented reality marker by using the calculated position and posture of the augmented reality marker; synthesizing the augmented reality content with the input image by using the calculated position and posture of the augmented reality content, and displaying the synthesized image; updating the position and posture of the augmented reality content by using a 6-axis sensor when the augmented reality marker is not recognized in the input image; and synthesizing the augmented reality content with the input image by using the updated position and posture of the augmented reality content, and displaying the synthesized image; wherein, when a size of the input image to be synthesized with the augmented reality content is smaller than a size of the augmented reality content, and when the augmented reality marker is not recognized in the input image, only a part of the augmented reality content is synthesized with the input image by: storing, as initial values, the position and posture of said part of the augmented reality content calculated at a point in time when the augmented reality marker is lastly recognized in the input image; calculating the position and posture changes of the 6-axis sensor from the point in time when the augmented reality marker is lastly recognized in the input image; and updating the position and posture of said part of the augmented reality content based on the initial values and the calculated position and posture changes of the 6-axis sensor.