A developing device has a sealing unit that seals a gap between an inner wall of a housing that stores developer and a blade that controls a thickness of developer attached on a developing roller. The sealing unit of the developing device includes a first sealing member inserted into a gap between the inner wall of the housing and an end portion of the blade, a second sealing member placed on the blade to contact the first sealing member, and a pressing member that presses the first and second sealing members so as to prevent gaps between the first and second sealing members. Therefore, the gap between the blade and the inner wall of the housing can be blocked efficiently without damaging the developing roller, and a stable and efficient developing process can be performed.