PURPOSE: To obtain distinct copying at all times irrespective of a ground color even if a ground tone of an original to be copied is varied in various ways, by constituting so that a copying parameter can be selected in accordance with the ground color of the original to be copied.

CONSTITUTION: An operation panel 14 is provided with a color display 21 for showing a ground tone, a density selecting button 19, etc. as well as prescribed operating buttons, and by this selection, developing bias voltage is controlled. The bias voltage is set lower by 100V and higher by 100V by "dark" and "light" of the button 19, respectively. In case of only said result, however, the background potential also becomes high in case of a color ground original and distinct copying is not obtained. Therefore, weighing is executed by the color display 21 of a button 20. That is to say, red, blue, yellow and green of the button 20 are selected so that the bias voltage becomes 200V, 150V, 100V and 250V higher than usual. It is also possible to vary the exposure quantity, or both the bias voltage and the exposure quantity instead of varying the bias voltage.