Abstract: A screw pressing type filtering apparatus that discharges a treated substance having a constant water content, including a screw rotatably provided inside a substantially cylindrical filter body so that a substance supplied to the space between the screw and the filter body is filtered by the filter body while being conveyed by the rotating screw. The apparatus further includes: a back pressure valve installed in the discharge port of the filter body to restrict the discharge amount of the filtered substance, an air cylinder that supplies a pressure to the back pressure valve, a valve opening detector which detects the opening amount of the back pressure valve relative to the discharge port, and a control section which controls the air cylinder based upon the detection results of the valve opening detector so that a pressure that corresponds to the degree of opening is supplied to the back pressure valve.