1. A depth of field adjustment device for an automatic document feeder, the automatic document feeder having a guiding groove, the guiding groove having a scanning window at an appropriate position, the scanning window is provided an optical chassis to proceed an image picking up on an object to be scanned that passes through the guiding groove, comprising: an adjustable suppressing mechanism, is capable of corresponding to the scanning window at an appropriate position of the guiding groove, and the located height of the object to be scanned that passes through the scanning window adjusted by the adjustable suppressing mechanism.