The invention has now found out why ghost light occurs in an imaging optical system using a decentered reflecting prism for the purpose of reducing ghost light. The invention provides an imaging optical system comprising at least one decentered prism 10 having a rotationally asymmetric reflecting surface, at least located on an image side of an aperture stop. The prism 10 comprises three or more surfaces including an entrance surface 11, a reflecting surface 12 and an exit surface 13. The reflecting surface has an effective area A+B+C, and a reflection-enhancing member is applied onto an area B+C except a sub-area A at which an effective light beam is totally reflected.