The characteristics of continuous-feed production laser printers are wide-format continuous paper printing, very high print speed, and heavy-duty use.These characteristics cause large machine size. In addition, large paper width requires large printer depth. To minimize the number of steps in the imaging process, a thin and wide developing unit is required. Furthermore, higher print coverage has recently become necessary. This, however, causes instability in toner concentration. It is therefore also important to maintain stability of toner concentration by correctly designing its control sequence.To obtain uniform developer flow in very narrow spaces, analysis method has been developed. It is applied to the design in this study.For obtaining smooth flow in slim (thin and wide) developing unit, combinations of two direction flows, which are quickly circulating developer in a cross section for a photoconductor movement and slower mixing with agree for its perpendicular direction, is needed. To achieve uniform developer flow rate and toner concentration in the imaging area, setting the optimum ratio of the two direction ratio flow rates is found to be an important factor. On the other hand, for obtaining stable toner concentration in a high coverage print, toner feed control is examined by using the analysis method. Employing PID control sequence attains sufficiently high stability.A design concept for the ultra-slim developing unit is reported.