In an image forming apparatus, in each of a cyan image forming unit, a magenta image forming unit, a yellow image forming unit, and a black image forming unit, during a non-development period during which no toner is discharged out of a development unit, when toner in the development unit is agitated with an agitation roller, every time the drive time of a development motor for driving the agitation roller is counted, the drive time is stored on an accumulative basis in an EEPROM; with respect to the developer unit of which the drive time has become equal to or longer than a predetermined period, a predetermined amount of toner is supplied thereto and the predetermined amount of toner is discharged therefrom, and thereafter the stored drive time is reset.