PURPOSE: To detect not only the toner concentration but also the entire part of the developer which determines the quality of images by providing the first electrode which also serves as a developing sleeve and the second electrode which opposes to said first electrode and serves also as a scraper sleeve.

CONSTITUTION: The first electrode 2 which is made to serve also as a developing sleeve is provided in opposition to a photosensitive drum 1. The second electrode 4 which is made to serve also as a scraper sleeve is provided in proximate opposition near the magnet being S1 pole to said first electrode 2 and is provided with such a magnet 5 being N3 pole as the one by which the magnetic force as a scraper means from above the first electrode 2 is exhibited. The elctric field by a bias power source 7 acts between the first electrode 2 and second electrode 4 and current flows to the circuit by a bias power source 7 on account of the transfer of the toner. If this current is detected, not only the toner concentration but also the developing capacity of the developer corresponding to the image quality may be detected.