Imaging optical system including a first optical unit (1) having positive refractive power for making an image at the enlargement-side of the imaging optical system and an intermediate image (3) conjugate to each other, and a second optical unit (2) having positive refractive power for making the intermediate image (3) and the image plane (5) at the reduction-side of the imaging optical system conjugate to each other, wherein when the focal length of the first optical unit (1) is denoted fF and the focal length of the second optical unit (2) is denoted fR, the inequalities 0