the invention provides an apparatus for developing an electrostatic latent image on a photoreceptor with developer . in the apparatus , there are provided a housing having an open end facing the photoreceptor ; a developing device including a sleeve , disposed in the housing in the vicinity of the open end , for supplying developer onto the photoreceptor through the open end ; feeder , disposed in the housing at a position further from the open end than the sleeve , for feeding developer to the sleeve ; a ridge - like protrusion , provided on a floor of the housing at a position beneath the sleeve , for preventing developer fed by the feeder from passing through between the sleeve and the floor .