The object is to provide an afocal zoom lens system with the f-number of about 4 or less capable of performing vibration reduction correction. The system includes, in order from an object, a first group G1 being positive, a second group G2 being negative, a third group G3 being positive, and a fourth group G4 being positive. Zooming is performed by moving the second and third groups along the optical axis. The fourth group G4 is composed of a front group G4F being positive, a middle group G4M being negative, and a rear group G4R being positive. Vibration reduction is performed by shifting the middle group G4M perpendicular to the optical axis. The first group G1 consists of a first front group G1f being fixed, a first middle group G1m being movable, and a first rear group G1r being fixed. Focusing is performed by moving G1m along the optical axis.