the invention provides a stereoscopic imaging optical system that has a total optical length enough short to be well fit for electron image microscopes . the stereoscopic imaging optical system comprises , in order from its object side , one objective lens ob and a plurality of zoom imaging optical systems zi . each zoom imaging optical system zi comprises , in order from its object side , a positive first group g 1 , a negative second group g 2 , an aperture stop as , a positive third group g # and a positive fourth group g 4 . the second group moves on an optical axis for zooming , and the fourth group moves on the optical axis in association with the second group for correcting an image position fluctuation incidental on zooming . conditions and are satisfied : condition is concerned with the imaging magnification at a low zoom ratio end of the whole of the groups on an image side with respect to the aperture stop , and condition is concerned with the ratios about a difference between the positions of the fourth group at a low zoom ratio end and a high zoom ratio end and a difference between the positions of the second group at a low zoom ratio end and at a high zoom ratio end .