A developing unit containing developer including carrier and toner and developing a latent image formed on an image carrier is disclosed. The developing unit includes multiple conveyor members configured to convey the contained developer in respective longitudinal directions so as to form a circulation channel; a supply part configured to supplement the carrier in the developing unit; a discharge part configured to discharge a first part of the contained developer outside the developing unit; and a bypass channel configured to cause a second part of the developer to return to the upstream side of the circulation channel without passing a position where the discharge part is provided.