PURPOSE: To obtain a zoom system image forming device for easily varying horizontal magnification and focusing at zooming and having high accuracy in zooming and obtaining an accurately focused image.

CONSTITUTION: The horizontal magnification at 100%, 50% and 200% and the number of steps for obtaining the focused state are respectively stored in a storage means 61. The other zooming position is calculated by performing the inverse operation of the proper value of the optical path length correction coefficient of a lens L at 100%, 50% and 200% stored in the storage means 61, and a pulse motor 40 is moved only by the amount of the necessary pulses so as to vary a copying magnification and a lens unit 9 is moved to decide each zooming position, and also, the pulse motor 50 is moved only by the amount of the necessary pulses to focus, and then, a 3rd carriage 12 is moved to focus.