A developing unit is provided that includes a housing to store toner, a developing member disposed proximate to the housing to rotate around a first rotation axis, and opposite to the photosensitive medium, and a cylindrical regulating member to rotate about a second rotation axis and to regulate the toner supplied to the developing member to form a uniform toner layer on the developing member, wherein the first and second rotation axes form a predetermined offset angle .theta. with each other. The angle satisfies the following formula: 0.25.degree..ltoreq..theta..ltoreq.0.71.degree.. The regulating member is arranged so that a center of each end portion of the regulating member which contacts the developing member is offset with respect to the first rotation axis of the developing member by an eccentric distance M, which satisfies the following formula: 0.5 mm.ltoreq.M.ltoreq.1.3 mm.