PURPOSE: To continuously obtain the preferable density of a copied image just after current replacement until next replacement by setting toner concentration lower than a predetermined controlled target value when developer is replaced and speedily replenishing a lacking amount of toner in terms of the controlled target value during warm-up just after the replacement.

CONSTITUTION: When developer is replaced, its concentration is set lower than the controlled target value, and during warm-up just after said replacement a lacking amount of toner is speedily replenished. Consequently, said toner rubs carriers only during the warm-up period. However, the toner cannot be sufficiently charged, and the entire toner becomes poor in charge quantity when development starts just after the developer has been replenished. Accordingly, the quantity of toner supplied to a photosensitive body increases as much as the charge quantity decreases, whereby deterioration in the density of a copied image just after toner replacement is restrained, and the desired density of a copied image can be obtained just after toner replacement.