a lens support unit 31 a sheet separating member 46 a main motor 80 and a fan motor 81 fig3 and 4 in combination show a control system included in the embodiment as shown the control system includes two cpus central processing units a and b which are assigned to sequence control and operation control respectively the cpus a and b are interconnected by a serial interface rs232c the sequence cpu a sets and outputs sheet transport timings and conditions relating to image formation connected to the cpu a are a sheet size sensor sensors response to sheet discharge and registration the duplex copy unit a high tension power source unit drivers including relays solenoids and motors the sorter unit the laser beam scanner unit etc the sheet size sensor senses the size and orientation of sheets stacked on a cassette and generates an electric signal representative of the result the other sensors include sensors responsive to oil toner and other supplies and sensors responsive to mechanical errors including a door open state and the blow out of a fuse the duplex copy unit includes a motor for positioning sheets in the widthwise direction a sheet feed clutch a solenoid for changing a transport path a sheet sensor a side fence home position sensor and sensors relating to sheet transport the high tension power source unit applies a particular high voltage to each of the main charger transfer charger separation charger and bias electrode for development by a duty derived from.