PURPOSE: To avoid the effect on a photodetector, of the light reflected with a color toner on a conductive mask by dividing a conductive member applied to a conductive mask into a light emitting side and a light receiving side and applying DC bias same in polarity as the color toner to the conductive member on the light emitting side.

CONSTITUTION: AC square waves same in polarity as a color toner are applied to a conductive member 13a on the light emitting side of a conductive mask 11, and the toner is not attached to the part of the conductive mask 11 on the light receiving side. On the other hand, AC square waves are applied to the conductive member 13b on the light receiving side, and when voltage same in polarity as the toner is applied, the mask 11 is kept clean, but the moment voltage opposite in polarity to the toner is applied, the toner and a carrier having so far attracted each other are separated, and the toner is attached only to the surface of the member 13b. The light emitted from a light emitting element 9a is transmitted through a transparent glass plate 12 free from the attached toner, reflected with a conductive probe 8, again transmitted through the glass 12 with the toner attached to it, and falls on a photodetector 9b. Thus, the concn. of the toner can be fetched as an electric signal.