An imaging optical system for imaging imagewise information provided on a surface of an original, upon a surface of an image pickup device such that the imagewise information is read based on signals from the image pickup device, the imaging optical system including four off-axial reflection surfaces which are asymmetric within a sub-scan section, wherein, when a combined power, in a main-scan direction, of two off-axial reflection surfaces of said four off-axial reflection surfaces which are optically close to an original side is denoted by Mf and a combined power, in the main-scan direction, of two off-axial reflection surfaces of the four off-axial reflection surfaces which are optically close to an image pickup device side is denoted by Mr, relations −0.3