An automatic document feeder, made a part of a two-side copying machine which selectably operates in one-side or two-side copying mode of operation, has a pre-feeder which advances a document to be copied next from its original position to a waiting position before reaching a copying position on a document table. A selecting device is further included for controlling the pre-feeder such that the pre-feeder does not function when the second surface of a document is being copied in two-side copying mode of operation.