An object of the present invention is to provide: a zoom lens system that has a high resolution, high capability of compensating curvature of field, a high zoom ratio of, for example, 3 or greater, a reduced weight, and a reduced overall optical length at the time of non-use; and an imaging device and a camera that employ this zoom lens system so as to have a reduced thickness and excellent portability as well as high performance. The zoom lens system forms an optical image of an object with variable magnification and, in order from the object side to the image side, comprises: a first lens unit having negative optical power; a second lens unit that has positive optical power and that contains a bi-convex lens element composed of a resin material and serving as the most image side lens element; and a third lens unit having positive optical power. Then, the lens units move respectively along the optical axis in such a manner that intervals between the individual lens units should vary so that the variable magnification is achieved. Further, the condition |(R2mi1+R2mi2)/(R2mi1−R2mi2)|≦1.0 (R2mi1 is a radius of curvature on the object side of the most image side lens element of the second lens unit, while R2mi2 is a radius of curvature on the image side of the most image side lens element of the second lens unit) is satisfied. The imaging device and the camera employ this zoom lens system.