PURPOSE: To detect the amt. of the color toner bonded to an image carrier which high optical accuracy especially in a high bonding region wherein the surface of the image carrier is perfectly covered with toner.

CONSTITUTION: The toner pattern image formed on an image carrier 1 is irradiated with the light of a light emitting element 2 and an image forming condition is controlled by the result obtained by detecting the reflected light thereof by a photodetector 3. A light emitting element 2 and the photodetector 3 have directional characteristics and the point P where the optical axes of the light emitting element 2 and the photodetector 3 cross each other at a right angle is present in the vicinity of the surface of the image carrier 1 and the optical axis plane S1 containing an optical axis with respect to the normal line (h) at the point P is inclined by an angle under a predetermined condition.