An image-taking optical system according to the present invention for forming an image of a two-dimensional object on an image-taking surface comprises a plurality of reflecting surfaces which reflect light from the two-dimensional object sequentially and guides the light to the image-taking surface, and a reference axis passing through the center of the pupil of the image-taking system and the center of the optical image formed on the image-taking surface is inclined with respect to the normal line of the two-dimensional object.The present invention provides an image-taking optical system which achieves a reduction in size, ensures a ratio of an amount of peripheral light, allows correction of trapezoidal distortion of a formed image, and is suitable for oblique image-taking.