Abstract: A reciprocating carrier-type photocopy machine having a photoelectric sensor for controlling a copy paper cut-off knife, to conform automatically the size of the copy paper to the size of the original document. A light emitter and photoelectric sensor are mounted beneath a glass platen over which a carrier, which contains the original document, moves for exposing the document to the reproducing mechanism contained beneath. An elongated rectangular mirror is mounted on the bottom surface of the carrier lid for reflecting the emitted light back to the sensor. The original document, when placed in the carrier, blocks the emitted light from being reflected from the mirror to the sensor until the trailing edge of the document passes the emitted light. The reflected light, upon passage of the document, then actuates the photoelectric sensor, which in turn actuates a solenoid of a copy paper cut-off knife to sever the copy paper from its supply roll.