PURPOSE: To vary the density of a copy image speedily and to reduce troubles such as the burning of an illumination lamp by controlling a photosensitive body bias applying means according to the output of an original density detecting means and decreasing a photosensitive body bias voltage as mean original density increases.

CONSTITUTION: A photodiode 14 provided by a lens 9 detects the quantity of reflected light (in inverse proportion to mean original density) from a specific range in an original breadthwise direction and its output signal is inputted to a microcomputer 16 through an amplifying circuit 15. The microcomputer 16 controls the photosensitive body bias applying means 12 on the basis of the input signal so that the photosensitive body bias voltage drops as the mean original density decreases. Therefore, copy image density is varied speedily according to the mean original density. Further, the illumination lamp is applied with a constant voltage so that the quantity of light is constant, so troubles such as its breaking of wiring are reduced.