A developing device includes a casing configured to store a developing agent and a developing agent carrier configured to carry the developing agent in the casing. The developing agent carrier is configured to rotate about an axis extending in a first direction. The developing device further includes a layer-thickness regulating member configured to regulate a thickness of a layer of the developing agent carried on the developing agent carrier and a first seal member disposed between the layer-thickness regulating blade and the casing. The first seal member extends along the first direction. The casing includes a first regulating portion configured to regulate a movement of the first seal member in the first direction by contacting a surface of the first seal member non-parallel to the first direction. The first regulating portion may include protrusions and/or indentations to regulate the movement of the first seal member.