An anti-shake device for positioning an image-capturing unit in an optical instrument. A base includes a shaft pillar. A clamp unit includes a first clamping arm and a second clamping arm. The first and second clamping arms pivot to the shaft pillar of the base and detachably clamp the image-capturing unit. A drive unit is connected to the base and includes a driven member driving the first and second clamping arms of the clamp unit to rotate and shifting between a first mode and a second mode. The first and second clamping arms are separated from each other and the image-capturing unit when the driven member is in the first mode. The first and second clamping arms close and clamp the image-capturing unit when the driven member is in the second mode.