An optical system comprising: an optical element that is made of a transparent medium that is rotationally symmetric relative to the central axis with a refractive index greater than 1; a front group (Gf); a back group (Gb) arranged at the side of the image plane relative to the front group (Gf); and an aperture arranged between the front group (Gf) and the back group (Gb), wherein the transparent medium has a first transmissive surface arranged at the outermost periphery relative to the central axis, a first reflective surface arranged at the side of the central axis relative to the first transmissive surface, a second reflective surface arranged at the side opposite to the image plane relative to the first reflective surface and a second transmissive surface arranged at the image plane side relative to the second reflective surface, that the flux of light entering the transparent medium goes into the transparent medium by way of the first transmissive surface so as to be reflected to the side opposite to the image plane by the first reflective surface and then to the image plane side by the second reflective surface to form a substantially Z-shaped optical path before going out from the transparent medium at the image plane side by way of the second transmissive surface in the order of forward ray tracing and that the optical path is formed only at a side relative to the central axis, the optical element is arranged in the front group (Gf) to form an image of an object arranged so as to surround the central axis or project an image of the object in a radial direction from the central axis, the optical system being characterized in that the first reflective surface has its concave surface directed to the aperture, and in that the optical system does not form any intermediate image on the optical path including a cross section including the central axis thereof and forms an image of an annular object around the central axis through the front group (Gf), the aperture, and the back group (Gb). an optical element that is made of a transparent medium that is rotationally symmetric relative to the central axis with a refractive index greater than 1; a front group (Gf); a back group (Gb) arranged at the side of the image plane relative to the front group (Gf); and an aperture arranged between the front group (Gf) and the back group (Gb), wherein the transparent medium has a first transmissive surface arranged at the outermost periphery relative to the central axis, a first reflective surface arranged at the side of the central axis relative to the first transmissive surface, a second reflective surface arranged at the side opposite to the image plane relative to the first reflective surface and a second transmissive surface arranged at the image plane side relative to the second reflective surface, that the flux of light entering the transparent medium goes into the transparent medium by way of the first transmissive surface so as to be reflected to the side opposite to the image plane by the first reflective surface and then to the image plane side by the second reflective surface to form a substantially Z-shaped optical path before going out from the transparent medium at the image plane side by way of the second transmissive surface in the order of forward ray tracing and that the optical path is formed only at a side relative to the central axis, the optical element is arranged in the front group (Gf) to form an image of an object arranged so as to surround the central axis or project an image of the object in a radial direction from the central axis, the optical system being characterized in that the first reflective surface has its concave surface directed to the aperture, and in that the optical system does not form any intermediate image on the optical path including a cross section including the central axis thereof and forms an image of an annular object around the central axis through the front group (Gf), the aperture, and the back group (Gb).