A developing device for an image forming apparatus may include a seal and a casing to prevent leakage of developer. In some examples, the seal member may include multiple side surfaces while the casing may include projections that may abut and contact various surfaces of the seal member. According to one arrangement, the casing may include first, second and third protrusions, where each protrusion is configured to abut and contact a corresponding surface of the seal member. Additionally, the second projection may protrude in a position inside of the first projection while the third projection may protrude in a position between the first and second projections.