PURPOSE: To detect toner density under no influence of the characteristics of a photosensitive material because a toner image is formed while charges given by an electrostatic charger are held as they are, by providing an insulator area to part of the photosensitive material and forming the toner image therein and measuring its density.

CONSTITUTION: The density of the toner image in the insulator area 12 is measured to decide on whether the density of a developer is higher or lower than a reference value accurately. A photosensor is used to detect the density of the toner image in the area 12 at specific timing. A toner image density signal 17 is supplied to a comparator 18, which compares the voltage level with a reference voltage signal 21 outputted by a reference voltage generator 19. The comparator 18 generates a toner supplement signal 22 when judging that the reflection density of the toner image in the area 12 is lower than the reference value. A toner dispenser driving circuit 23 once supplied with the toner supplement signal 22 replenishes toner to the developer. Consequently, the density of the developer is controlled accurately.