An object is to provide: a zoom lens system that has a reduced overall length, a high resolution, and a variable magnification ratio as high as 9 or greater and that is satisfactorily adaptable for wide-angle image taking where the view angle at a wide-angle limit is 70° or greater; an imaging device employing this zoom lens system; and a thin and compact camera. The zoom lens system has a plurality of lens units each consisting of at least one lens element, and, in order from the object side to the image side, comprises a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power, a fourth lens unit and a fifth lens unit. In zooming from a wide-angle limit to a telephoto limit at the time of image taking, at least the first lens unit and the second lens unit move in an optical axis direction, while the fourth lens unit and the fifth lens unit each consists of one lens element. An imaging device and a camera employ this zoom lens system.