An imaging optical system, comprising a first reflector, a second reflector, and a third reflector that are arranged in this order on an optical path of incident ray, in which the arrangement of the three reflectors (1, 2, 3) is such that the following conditions are fulfilled: the three reflectors are arranged so as not to obscure the incident ray, wherein the first reflector and the third reflector are arranged so as to oppose each other; light beams reflected by the three reflectors form an image plane; a convex mirror is used for one of the first reflector and the third reflector and a concave mirror is used for the other thereof; in terms of a central chief ray, which is defined as a chief ray of light beams forming an image at a center of the image plane, vertexes of a triangular bipyramid are defined by a point on the central chief ray that is incident to the first reflection surface, a reflection point of the central chief ray on the first reflection surface, a reflection point of the central chief ray on the second reflection surface, a reflection point of the central chief ray on the third reflection surface, and an image forming point of the central chief ray form on the input image plane, and the central chief ray incident to the first reflection surface and the central chief ray reflected by the third reflection surface are forming opposing ridgelines of the triangular bipyramid; in terms of the central chief ray defined as a chief ray of light beams forming an image at a center of the image plane, when an angle formed by light beams incident to the second reflection surface and reflected light beams is represented as Φ2 in degrees, a distance between the third reflection surface and an image plane is represented as d3, and a focal length of the imaging optical system is represented as f, the condition Φ2>180-20d3/f is satisfied; a reference plane containing three points composed of the reflection point of the central chief ray on the first reflection surface, the reflection point of the central chief ray on the second reflection surface, and the reflection point of the central chief ray on the third reflection surface coincides with a bonding plane between two triangular pyramids that form the triangular bipyramid; and the central chief ray of the light beams incident to the first reflector and the central chief ray of the light beams reflected by the third reflector do not exist on said reference plane. the three reflectors are arranged so as not to obscure the incident ray, wherein the first reflector and the third reflector are arranged so as to oppose each other; light beams reflected by the three reflectors form an image plane; a convex mirror is used for one of the first reflector and the third reflector and a concave mirror is used for the other thereof; in terms of a central chief ray, which is defined as a chief ray of light beams forming an image at a center of the image plane, vertexes of a triangular bipyramid are defined by a point on the central chief ray that is incident to the first reflection surface, a reflection point of the central chief ray on the first reflection surface, a reflection point of the central chief ray on the second reflection surface, a reflection point of the central chief ray on the third reflection surface, and an image forming point of the central chief ray form on the input image plane, and the central chief ray incident to the first reflection surface and the central chief ray reflected by the third reflection surface are forming opposing ridgelines of the triangular bipyramid; in terms of the central chief ray defined as a chief ray of light beams forming an image at a center of the image plane, when an angle formed by light beams incident to the second reflection surface and reflected light beams is represented as Φ2 in degrees, a distance between the third reflection surface and an image plane is represented as d3, and a focal length of the imaging optical system is represented as f, the condition Φ2>180-20d3/f is satisfied; a reference plane containing three points composed of the reflection point of the central chief ray on the first reflection surface, the reflection point of the central chief ray on the second reflection surface, and the reflection point of the central chief ray on the third reflection surface coincides with a bonding plane between two triangular pyramids that form the triangular bipyramid; and the central chief ray of the light beams incident to the first reflector and the central chief ray of the light beams reflected by the third reflector do not exist on said reference plane.