A developing device comprising: a developing roller; a casing that includes a developer storage unit; a roller supporting unit and an opening; and a seal member that is mounted on the casing outside the opening in the central axis direction and faces an end portion of the developing roller, the seal member configured to rub against the end portion of the developing roller when the seal member is elastically deformed, wherein the casing includes: an outer projection that is provided along a moving direction of the developing roller to abut on an outer end portion of the seal member, the outer projection protruding in a position which is inside the roller supporting unit; and an inner projection provided along the outer projection so as to abut on an inner end portion of the seal member, the inner projection protruding in a position which is outside the opening.