An imaging optical system includes a bending optical element which bends an object-emanating light bundle, a post-bending lens system on a post-bending optical axis defined by the bending optical element, and an image sensor. An effective optical surface of a large-diameter lens element, having a greatest axial light bundle effective radius, is formed into a non-circular shape by making a length of the effective optical surface from the post-bending optical axis toward a side opposite from the object side smaller than the axial light bundle effective radius, with reference to the axial light bundle effective radius lying on a plane which extends orthogonal to a plane including both the post-bending optical axis and a pre-bending optical axis of the imaging optical system and includes the post-bending optical axis.