A document separating mechanism for an automatic document feeder for separating and feeding the bottommost document of documents laid on document bearing means. A separating roller mechanism has a plurality of rollers provided at spaced locations in an axial direction, and a separating belt mechanism is disposed opposite and above the separating roller mechanism. The separating belt mechanism is composed of a driving roller and a driven roller disposed in parallel at spaced locations, a wide separating belt looped over the driving roller and the driven roller and pressed against the separating roller, and a tension imparting mechanism for exerting tension on the separating belt.