An optical system includes: a main mirror (11) having a shape of a portion of a convex paraboloid which includes an opening in a center and is rotationally symmetric; a second-reflection mirror (12) which further reflects light reflected by the main mirror (11), and has a shape of a portion of a concave paraboloid which is rotationally symmetric; at least one lens which forms an image of the light reflected by the second-reflection mirror (12); and a lens barrel (14) holding the at least one lens, and a position of a front principal point of the at least one lens coincides with a focal position of the second-reflection mirror (12), and an optical axis of the at least one lens is tilted with respect to a rotational axis of each of the convex paraboloid and the concave paraboloid.