An image projection optical system introduces light from an image-forming element by a zoom lens via an optical element. The optical element satisfies .SIGMA.(Di.times.Ndi)/fw>4.0. The zoom lens includes, in order from a magnification side, first and second lens units, and lens units disposed closer to a reduction side than the second lens unit. The first lens unit is a negative lens unit. The lens units disposed closer to the reduction side than the second lens unit satisfy |dn/dt|<1.0.times.10.sup.-5. Magnification-side positive lens elements disposed closer to the magnification side than a negative lens element whose effective diameter is smallest in the lens units disposed closer to the reduction side than the second lens unit satisfy {9.0.times..SIGMA.(1/fpA)+5.0.times..SIGMA.(1/fpB)+0.5.times..SIGMA.(1/fp- C)}/.SIGMA.(1/fp)<1. Reduction-side positive lens elements disposed closer to the reduction side than the negative lens element whose effective diameter is smallest satisfy {9.0.times..SIGMA.(1/fpA)+0.3.times..SIGMA.(1/fpB)+2.0.times..SIGMA.(1/fp- C)}/.SIGMA.(1/fp)<1. Chromatic aberration is corrected and focus variations by temperature changes are reduced.