A zoom lens of simple structure is disclosed comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive refractive power, wherein these three lens units each are constructed from a single block, and zooming from the wide-angle end to the telephoto end is performed by moving the first and third lens units axially toward the object side independent of each other.