Provided is a zoom lens optical system suitable for cameras using solid state imaging devices, which is designed compact, has a high zooming ratio and telecentricity, and compensates for color aberration. The zoom lens optical system includes a first lens group having a positive refractive power, for performing zooming, a second lens group having a negative refractive power, for performing zooming, a third lens group having a positive refractive power, for performing zooming, and a fourth lens group having a positive refractive power. The first, second, third, and fourth lens groups are sequentially arranged from an object and satisfy the following equations:.gtoreq. ##EQU00001## .ltoreq..ltoreq. ##EQU00001.2## where f.sub.w denotes the overall focal length of the zoom lens optical system at the wide angle position, f.sub.T denotes the overall focal length of the zoom lens optical system at the telephoto position, and L.sub.III denotes the amount of movement of the third lens group from the wide angle position to the telephoto position. Zooming from the wide angle position to a telephoto position is performed so that the distance between the third lens group and the fourth lens group increases.