An auto focus device of a mobile communication apparatus is disclosed. The auto focus device comprises a voice coil motor, a first driving circuit and a second driving circuit. The voice coil motor comprises a floating movable device and a lens in the floating movable device. The first driving circuit drives the voice coil motor so that the floating movable device moves inside the voice coil motor to zoom in/out and focus the lens on image sensors when the camera function is activated. The second driving circuit drives the voice coil motor to fix the floating movable device inside the voice coil motor and cover the image sensors.