To surely prevent the quality of an output image from degrading by determining the timing of implementing the calibration of image density, based on the driving time of a developing device.

A timer 90 for counting the driving time of a developing drive motor 44 is installed in the image forming apparatus and further the driving time of the developing drive motor 44 counted by the timer 90 is accumulated and stored in an EEPROM 93. A controller 94 implements the calibration of the image density, when it is confirmed that the driving time of the developing drive motor 44 accumulated and stored in the EEPROM 93 reaches the driving time of the developing drive motor 44 up to the implementation of the calibration of the image density stored in a ROM 96.