An electrophotographic copying apparatus which includes a thin sheet original transporting device having transporting rollers and disposed adjacent to a transparent platform for holding thick originals at the upper portion of the copying apparatus, and an image forming optical system moving device for moving the optical system from a scanning starting position for thick original copying to a stationary optical system position for thin sheet original copying in association with a change-over of operation from copying a thick original to copying a thin sheet original in order to eliminate disadvantages inherent in exposures in conventional copying apparatuses, and the distance between first and second reflecting mirrors of the optical system at the scanning starting position for copying thick originals is set to be less than that when the optical system is in the stationary position for reduction of the size of the copying apparatus.