A developer storage case includes: a storage case that includes a discharge opening formed in the horizontal direction side surface thereof, that is formed in a flat shape having a horizontal direction width larger than the vertical direction height thereof, and that stores a developer therein; a delivery member that has a center of rotation disposed shifted to the discharge opening, that is rotatably supported on one end wall of the storage case, and that delivers the developer stored within the storage case; and a drive transmission member that is supported on the end portion of the delivery member, and that drives and rotates the delivery member.