Abstract: To provide a compact imaging optical system using reflecting mirrors that can be equipped and used in a vehicle or the like. An imaging optical system according to the invention includes three reflecting mirrors (103, 107, 109) and configured, when an XYZ orthogonal coordinate system using an optical axis at the center of the field of view as Z-axis is determined, so that the optical axis at the center of the field of view and an optical axis of an image surface may be in parallel by changing orientation of the optical axis in a YZ section while maintaining the orientation of the optical axis in an XZ section, wherein a shape of a YZ section of at least one reflecting mirror is made asymmetric with respect to Z-axis of local coordinates of each reflection surface for reducing aberration.