PURPOSE: To project a reflected light exactly corresponding to the concn. of a developer on a photosensitive body, on a photodetector without being affected by external scattered lights and to prevent a transparent glass plate from being stained with a color toner by using the light of the complementary color of the color toner to be detected in concn. as the light source of a light emitting element, and applying DC bias same in polarity as the color toner to a light transmitting conductive substance layer formed on the transparent glass.

CONSTITUTION: DC bias same in polarity as the color toner is applied to a conductive member 6 formed on a transparent glass plate 5 to prevent attachment of the color toner to the glass 5 and stains of a concn. detector 4, too. The light emitted from a complementary color light emitting element 4a installed in this detector 4 is transmitted through the glass 5, reflected with an original image formed with a color toner of a photosensitive drum 1, and projected on a photodetector 4b. Since the incident light atm. of the light having the complementary color of the color toner incident on the photodetector 4b corresponds to an amt. of color toner, i.e. the color toner concn. of the developed image on the drum 1, the concn. of the color toner can be fetched as an electric signal.