A developing device includes a developer bearer, a magnetic field generator, a casing having an opening, a developer conveyor to rotate inside a developer containing compartment below the developer bearer, and a developer regulator. A side wall of the casing defines a bottom end of the opening, and the developer regulator is disposed on the side wall. The side wall includes an upper end face facing the developer bearer below an axis of the developer bearer and a curved inner face along an orbit of rotation of the developer conveyor, extending from below the developer conveyor toward the upper end face. On a virtual plane perpendicular to the axis of the developer bearer, an intersection between a tangent line to an upper end of the curved inner face and the surface of the developer bearer is in a range from a tangential magnetic-flux peak and a closest approach point.