An imaging optical system includes a plurality of lens groups each moving such that spaces between each one of the plurality of lens groups change during a zooming. The imaging optical system conjugates a conjugate point on a magnification side and an intermediate imaging position, and conjugates a conjugate point on a reduction side and the intermediate imaging position. The imaging optical system includes a first lens group located at a furthest place on the magnification side and a rear group in this order from the magnification side toward the reduction side. The first lens group includes a field curvature correction lens group moving along an optical axis when an amount of a field curvature is changed, and a focusing lens group. The imaging optical system satisfies condition (4) below: |{(1−βcw2)×βcrw2}/{(1−βfw2)×βfrw2}|