PURPOSE: To exactly measure the density of an original at an image forming position of an original image, by providing a lens for forming the original image to a density detector, by combining it with the density detector, in addition to an optical system for leading a reflected ray of an original to a photosensitive body.

CONSTITUTION: On an optical path being separate from an optical system 2 for leading a reflected ray from an original, to a photosensitive body 3, a body 4 of an original density detecting device is installed. To one end side like a cylinder, of the body 4, a lens 5 is inserted, and on the other end side, a density detector 6 is installed at an image forming position of an original 2 projected by the lens 5. The original density detected by the density detector 6 is sent to a control system, and in accordance with the detecting signal, the quantity of light of an exposure lamp, the charging quantity of a charger or the developing bias, etc. are controlled, therefore, a copy of correct density is always obtained irrespective of the original density.