Abstract: An image stabilizer applicable to a monocular or binocular optical system. The stabilizer includes an erecting prism placed between an objective lens and an eyepiece to compensate for deterioration in a quality of an observed image caused by hands movement. The image stabilizer includes a gimbal suspension system having pivot shafts respectively extending to left and right sides of the optical axis of the optical system. The pivot shafts have a common pivot axis perpendicular to the optical axis. The gimbal suspension system has an erecting-prism holder supported by the pivot shafts, an angular velocity detector for detecting an angular displacement of the erecting prism, and a voice coil motor for pivoting the erecting-prism holder.