A developing device can supply sufficient amount of developing agent into a developing chamber regardless of residual amount of developing agent in a developing agent chamber. The developing chamber has a developing roller and a supply roller. The developing agent chamber is positioned below the developing chamber. A wall portion is positioned below and extending along a lower portion of the supply roller. A vertical conveyer is configured to convey the developing agent from the lower portion of the developing agent chamber to a position above the supply roller. The developing chamber has a reservoir positioned above the supply roller and configured to accumulate the developing agent conveyed by the vertical conveyer.