PURPOSE: To form a toner attached region for measurement on the surface of a photosensitive drum without receiving the fluctuation of the luminous intensity of an exposure lamp, by having a high degree of concentration for a reference chart which is set beside an original setting surface for formation of a toner image concentration measuring region.

CONSTITUTION: The volume of the light of a lamp 22 which is reflected by a reference chart 16 of high concentration is always small with no effect of the lamp light volume. Therefore, the toner image concentration produced at the image projecting region of the chart 16 on the surface of a photosensitive drum depends totally on the toner concentration of a developer. In this connection, the toner concentration is detected by a sensor 31 to control a toner supplying device 8. Thus it is possible to keep an optimum level of toner concentration. Under such optimum conditions, the concentration has a big displacement to show a fault of bias value if the development bias is not proper by simply setting the toner image of the chart 16 at the intermediate concentration.