To provide a compact imaging optical system in which occurrence of an asymmetrical aberration is reduced and the optical performance is not deteriorated significantly by constituting the system by off-axial reflection surfaces, and an image reading apparatus using the same. The imaging optical system of the present invention is characterized in that, in the imaging optical system for imaging image information of an object surface on a line sensor, all of a plurality of reflection surfaces existing in an optical path from the original surface to the line sensor are off-axial reflecting surfaces.