To provide an optical path tracking method, an optical path display method, an optical path display device, and a lens designing method by which an estimated optical path locus can sufficiently, accurately and exactly be grasped without any malfunction in a short time even when an optical path or lens form is changed.

By using the position coordinate of the optical path to be tracked, the coordinate of an intersection between light and each lens element is exactly calculated through processing using jointly a vector expression and the line segment expression of maximum linear distance within a drawing range (S4). Transmitted light, reflected light and scattered light required for tracking the optical path are found in the direction of 360° form an incidental light vector, boundary plane normal vector, incidental angle, refraction angle and difraction factor with respect to respective lens elements by adding the conception of full reflection in the vector expression expanding the rules of refraction (S5 and S6). Such processing is repeated for required times.