PURPOSE: To provide a zoom lens device constituted so that a lens is easily adjusted and capable of improving image forming performance.

CONSTITUTION: This device is provided with a zoom lens being a lens group for forming the image of an original on the surface of a photoreceptor drum at an optional magnification, a lens holder 24 being a lens placing member on which the zoom lens is placed, lens barrels being plural lens holding means for holding the lens, a cam plate being a lens moving means for moving the lens barrel, and a reference shaft 27 and a guide shaft 28 being a lens guiding means for guiding the movement of the lens barrel. In the device, a focal distance is varied by moving the lens barrel in an optical axis direction. The reference shaft 27 and the guide shaft 28 are supported by one lens barrel, and all other lens barrels (including a fixing lens barrel 23) are positioned by the reference shaft 27.