Provided is a compact and bright imaging optical system having a high resolution. The imaging optical system includes three reflectors composed of a first reflector (1), a second reflector (2), and a third reflector (3) that are arranged in this order on an optical path of incident ray so as not to block the incident light. In the imaging optical system, in which light beams reflected by the three reflectors form an image plane (4), a convex mirror is used for any one of the first reflector (1) and the third reflector (3) and a concave mirror is used for the other thereof, and vertexes of a triangular dipyramid (6) are defined in terms of a central chief ray (5) by an appropriate point on the central chief ray (5) that is incident to the first reflection surface, a reflection point of each central chief ray on the first to third reflection surfaces, and an image forming point of the central chief ray. A plane containing three reflection points of the central chief ray on the first to third reflection surfaces coincides with a bonding plane between two triangular pyramids forming the triangular dipyramid (6).