The invention concerns an optical system which is adapted to take full-360.degree. angle-of-view images from the surrounding scene or project images on the full-360.degree. angles of view, and which has compact size, reduced flare light and high resolving power. The optical system comprises a front unit 10 having two reflective surfaces 11, 12 rotationally symmetric about a center axis 1, and a rear unit 20 that is rotationally symmetric about the center axis and has positive power. The front unit comprises a first reflective surface 11 located facing away from an entrance pupil 6Y and a second reflective surface 12 located facing away from the first reflective surface 11. The entrance pupil 6Y in a section including the center axis is disposed between the outer periphery of the first reflective surface 11 and the outer periphery of the second reflective surface 12. A light beam 2 incident from afar travels through the front unit 10 and the rear unit 20 in order, and is imaged at a position of an image plane 30 off the center axis. In the section including the center axis, the entrance pupil 6Y is positioned off the center axis, and in a section that is orthogonal to a plane including the center axis and includes a center ray of that light beam, a entrance pupil 6X is positioned on the center axis. The rear unit 20 comprises at least two groups.