An image projection optical system introduces light from an image-forming element by a zoom lens via an optical element. The optical element satisfies Σ(Di×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|