PURPOSE: To maintain an image having as good quality as possible and to prevent the device from being stopped even when the density of a reference toner image is abnormal by discriminating whether a condition for forming the appropriate reference toner image on an endless carrier is satisfied and controlling a toner supply amount based on the density information of the image when the condition is not satisfied.

CONSTITUTION: The reference toner image 2 is formed on the endless carrier 1 which is driven to be rotated and the density of the reference toner image 2 is detected by a density detection means 3, whereby 1st control where the toner supply amount is controlled and 2nd control where the toner supply amount is controlled based on the image density information 4 are executed. Then, a discrimination means 5 discriminates whether or not the condition for forming the appropriate reference toner image 2 on the carrier 1 is satisfied. In the case the discriminated result by the discrimination means 5 shows that the condition is not satisfied, the control means 6 switches the control to only the 2nd control.