An image stabilizer for stabilizing an image against unexpected vibrations or shaking of a zoom lens includes a diaphragm arranged to vary its full-open-aperture diameter according to a magnification varying (zooming) action from a wide-angle end position to a telephoto end position of the zoom lens for the purpose of preventing variations in light quantity which more conspicuously take place in the peripheral part than in the central part of an image plane when an image stabilizing action is performed.