is the toner density of the two component developer in the developing means container 40 m is detected by detecting the image of the patch referential toner image with the use of an optical toner density sensor unshown for example if it is determined that the toner density is below the abovementioned proper range a target level for the toner density ratio t d is increased in value to increase the amount by which toner is supplied to the developing device 4 from the toner hopper unshown toner supplying means as the developing device 4 is supplied with a fresh supply of toner the toner density ratio t d increases which in turns reduces in the amount of charge the toner in the developing device 4 thus as the developing device 4 is supplied with a fresh supply of toner a control is executed to keep constant the toner density relative to the preset development contrast vcont on the contrary if it determined that the toner density is above the abovementioned proper range the delivery of the toner to the developing device 4 is temporarily halted here u201c development contrast vcont u201d means the difference vdc u2212 v 1 between the potential level v 1 of the exposed areas of photosensitive drum 1 after the exposure of.