A stepwise variable zoom lens system according to the present invention has, in order from an object side, a first lens group of negative refracting power, a second lens group of positive refracting power, and a third lens group of positive refracting power. The first and third lens groups are constructed from a single block. The second lens group includes a positive first lens unit and a negative second lens unit which is also constructed from a single block. The first and third lens groups are fixed, and the second lens group is movable to be selectively disposed in several discrete positions. The focal lengths of the lens groups satisfy the following relations:

##EQU1##

##EQU2##

• wherein f₁, f₂ and f₃ denote the focal lengths of the first, second and third lens groups respectively, f_b denotes the back focal length of the stepwise variable zoom lens system, and z denotes the zoom ratio.