The disclosure describes a variable magnification optical mechanism in an image forming apparatus in which a varied image at a selected magnification of an original image is formed. A photosensitive body on which the varied image is exposed is moved at a definite speed (v) irrespective of the selected magnification (m) and optical means for transmitting the image of the original to the photosensitive body is driven at a speed ratio of v/m. The pre-running length (l) of the optical means is changed in accordance with the selected magnification (m) at a ratio of l/m so that coincidence of the original image and the variation image formed therefrom may be attained every time.