The decentering optical system comprises a decentering prism and a holding component. The decentering prism has two or more positioning portions at positions which sandwich an effective diameter of an optically functional surface held by the holding component. The holding component has a holding portion of positioning portions corresponding to the positioning portions of the decentering prism. Furthermore, on the optically functional surface of the decentering prism, two and more projected portions are arranged at a position which sandwich the effective diameter of the optically functional surface and is located at the outside of the positioning portion, and a projected portion is located inside of the positioning portion. The decentering prism is fixed for positioning at the holding component through the positioning portion, the holding portion of the positioning portion and the projected portions.