1. A method of calibrating a camera using projection invariable characteristic of a concentric circle in order to calibrate the camera through a calculation of camera's intrinsic parameters, the method comprises steps of; a) taking a plurality of images of a calibration pattern consisting of two or more concentric circles located at a same plane and having different radiuses at different angles to obtain projected images resulted from the projection of the concentric circles; b) calculating central points of the projected images using a given algorithm; and c) calculating an optimum principal point (u 0 , v 0 ) and focal length (f) of the camera using a nonlinear minimizing method based on a given principal point and focal length, which are the camera's intrinsic parameters, and the central point of the projected images calculated in the step b), wherein the concentric circle includes two circles and the step b) includes the steps of: d) calculating the central points (O) of the two projected images obtained from the step a) to obtain a straight line passing through the central points; e) calculating crossing points of the straight line and the two projected images; and f) calculating the central point of the projected images using a cross ratio depending on the central point (O) of the projected images, a given point (M ∞ ) infinitely separated from the central point (O) on the straight line passing through the central points (O), and the crossing points of the straight line and the projected images.