An image magnification control device for a camera having a photographic lens. A distance "X0" between a rear focal point of said photographic lens and an image taking plane of the camera is detected. Additionally, a defocusing amount "dx" corresponding to a distance between the image taking plane and a surface on which an image is formed by the photographic lens is detected. A magnification ratio .gamma. of a control focal length "f1" required to maintain an image magnification to a present focal length "f0", that is based on the distance "X0" and the defocus amount "dx" is then calculated in accordance with a predetermined equation. Thereafter, an amount to drive the photographic lens to maintain the image magnification, based on the magnification ratio .gamma. is calculated, and the photographic lens is driven to change a focal length of the photographic lens.