A developing unit has first and second partition walls and which divide the inside of a developer reservoir into first to third chambers parallel to the developing roller, and first to third mixers which are disposed in the first to third chambers. The first mixer supplies developer to the developing roller by stirring and carrying from one end side to the other end side. The second mixer feeds by stirring and carrying the toner replenished from a replenish device and developer fed from the first chamber to one end side of the first mixer. The third mixer feeds the toner collected from a collection mechanism to one end side of the first mixer by stirring and carrying at a speed lower than the second mixer.