12 and the cleaner 13 are assembled into a drum unit 2 which is removable along slide rails 3 and 4 toward the front i e toward the viewer of the drawing an optical system 20 comprises a light source 21 mirrors 22 23 24 25 and a lens 26 the light source 21 and the mirror 22 move at a speed v and the mirrors 23 24 travel at a speed v 2 whereby an original on a glass plate 27 is continuously scanned an optical image of the original is projected on the drum 10 charged to a predetermined potential by the sensitizing charger 11 and rotating in the direction of arrow a the latent image formed is then developed by the developing unit 12 to a toner image which is thereafter brought to a transfer station a detachable paper cassette 40 has accommodated therein sheets of copy paper which are sent out one by one by the rotation of a feed roller 42 in the direction of arrow b the copy paper sent out is turned upside down by a guide plate 43 and transported to the transfer station by a pair of timing rollers 44 for adjusting the timing of transport of the copy paper to register at the transfer station the leading end of the paper with the forward end of the toner image formed on the drum 10 at the transfer station the toner image is transferred to the copy paper by the discharge of the transfer charger 6 and the image.