A compact bent zooming imaging optical system characterized by sufficient aberration correction. The imagining optical system contains, in order from the object, a first lens group having positive refractive power, a second lens group with negative power which has a reflective optical element for bending the optical axis and lenses on each of the object side and the image side of the reflective optical element, and a following lens group. At least the aforementioned first lens group and following lens group are moved to change in the magnification. -0.6<.phi.1/.phi.2<-0.15 is satisfied wherein .phi.1 denotes the refractive power of the first lens group and .phi.2 indicates the refractive power of the second lens group.