A developing device has a housing including a toner container containing toner, and a developing unit. The developing unit includes a developing roller and a photoconductive drum, and a barrier member including a toner supply window. The barrier member is combined with the housing to separate the toner container and the developing unit from each other. The developing unit also includes a blocking film that is attached to the barrier member to block the toner supply window. An end of the blocking film is exposed outside the housing through an opening formed in a sidewall of the housing. A bottom surface and first and second side surfaces of the barrier member are attached to a bottom surface and first and second side surfaces of the housing by using a welding process, and a distance between the first and second side surfaces of the barrier member and a distance between the first and second side surfaces of the housing increase from bottom to top.