A developing device includes first-third sections, a developing roller, a partitioning wall and a toner concentration detector. The first-third sections have first-third screw augers respectively. The partitioning wall isolates the third section from the first section and the second section. A supply opening is formed in a portion of the partitioning wall opposing the first section. A toner concentration detector is disposed in the third section. The first screw auger conveys developer including toner to the developing roller, into the third section via the supply opening, and into the second section. The second screw auger conveys developer accommodated in the second section into the third section. The third screw auger conveys developer accommodated in the third section into the first section. Toner is replenished into the third section based on a detection result by the toner concentration detector.