a first section, a second section and a third section configured to accommodate developer including toner and carrier therein, and having a first screw auger, a second screw auger and a third screw auger respectively that rotate to agitate and convey the developer;

a developing roller that supplies developer to an image-carrying member on which latent images are formed in order to develop the latent image;

a partitioning wall extending in a vertical direction that isolates the third section from the first section and the second section, a first opening and a plurality of supply openings being formed in portions of the partitioning wall opposing the first section, a second opening being formed in a portion of the partitioning wall opposing the second section; and

a toner concentration detector disposed in the third section, and detecting concentration of toner in developer accommodated in the third section;

wherein the first screw auger conveys developer accommodated in the first section to the developing roller, into the third section via the plurality of supply openings opening, and into the second section,

wherein the second screw auger conveys developer accommodated in the second section into the third section via the second opening,

wherein the third screw auger conveys developer accommodated in the third section into the first section via the first opening, toner being replenished into the third section based on a detection result by the toner concentration detector.