To provide an image forming optical system for reading a document, capable of readily attaining a carriage integral type scanning system, in a document reading system for a digital copying machine, or the like, requiring high-speed properties and high resolution, and to provide an image reader using the image forming optical system for reading the document.

The image forming optical system includes a first off-axial reflecting surface, a second off-axial reflecting surface, a third off-axial reflecting surface, and a fourth off-axial reflecting surface in order from an object surface; a diaphragm is arranged in an optical path, between the second off-axial reflecting surface and the third off-axial reflecting surface, and in a sub-scanning cross section, the optical path of a light ray going from a document surface to the first off-axial reflecting surface and the optical path of a light ray traveling from the second off-axial reflecting surface to the diaphragm cross each other, and in other areas, crossing is not performed; and image information is image-formed on the image forming surface of a line sensor, without intermediate image formation and the product between the power P(mm-1) of the sub-scanning cross section of the off-axial reflecting surface and an interval S(mm), along a reference axis from the off-axial reflecting surface to the next off-axial reflecting surface is set properly.