Disclosed is a sludge processing system having a centrifugal separator concentrating the sludge and a furnace heat-treating the concentrated sludge. It allows automatic control in which the operational condition of the centrifugal separator operates under optimal conditions according to the operational state of the furnace. The control device sets a variable range of a differential speed between the bowl and screw conveyor of the centrifugal separator, and variably controls the differential speed so concentrated sludge convey torque of the screw conveyor achieves a torque corresponding to a moisture content requested by the furnace within the variable range, when the differential speed varies within a certain previously set range and the concentrated sludge convey torque of the screw conveyor maintains the torque corresponding to the moisture content requested by the furnace for a certain period of time being previously set, changes a control value of the centrifugal force and the torque.