A developing device includes a housing having an opening in a wall, a developing roller having a hollow interior portion, a magnetic pole structure disposed within the hollow interior portion of the roller, a gap forming member, and a blocking member. The gap forming member is located within, and spaced from, the housing at a location downstream, in the rotation direction of the developing roller, of the opening in the wall of the housing, forms a first gap with respect to the developing roller and forms a second gap with respect to the housing. The gap forming member has a first side facing the opening in the wall of the housing and a second side spaced inwardly of the housing, from the first wall. The blocking member is located in the first gap and at one of the first and second sides of the gap forming member.